How do I get rid of an iPhone security warning?

It’s a scam.

See >>> Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support

Don’t respond to, or engage with, the message. Certainly do not, under any circumstances, attempt to contact anyone using the contact details that may have been provided. Alerts of this nature are designed to scare the unwary into giving away sensitive information - or to fool you into doing something that you shouldn’t - usually to defraud you financially.

Just close the browser tab. If you are unable to close the associated alert/window, perform a forced restart of your iPad:

Force restart iPad (models with Face ID or with Touch ID in the top button)

If your iPad doesn’t have the Home button, do the following:

1. Press and quickly release the volume button nearest to the top button.
2. Press and quickly release the volume button farthest from the top button.
3. Press and hold the top button.
4. When the Apple logo appears, release the top button.

Force restart iPad (models with the Home button)

Press and hold the top button and the Home button at the same time. When the Apple logo appears, release both buttons.

Providing your iPad has been kept up-to-date with system software updates, you should not be overly concerned for your iPad being directly compromised by malware.

For older devices, no longer benefiting from regular security updates, or those that are not kept up to date when system software updates are available, the risk of an unpatched vulnerability being exploited increases. 

Threat Mitigation

The majority of threats to which you will be invariably exposed will surface via web pages or embedded links within email or other messaging platforms. Browser-based attacks can be largely and successfully mitigated by installing a good Content and Ad-blocking product. One of the very best and most respected within the Apple App Store - designed for iPad, iPhone and Mac - is 1Blocker for Safari.

https://apps.apple.com/gb/app/1blocker-for-safari/id1365531024

1Blocker is highly configurable - and crucially does not rely upon an external proxy-service of dubious provenance, often utilised by so-called AntiVirus products intended for iOS/iPadOS. Instead, all processing by 1Blocker takes place on your device - and contrary to expectations, Safari will run faster and more efficiently.

Unwanted content is not simply filtered after download (a technique used by basic/inferior products), but instead undesirable embedded content is blocked from download. The 1Blocker product has also recently introduced its new “Firewall” functions - that are explicitly designed to block “trackers”. Being implemented at the network-layer, this additional protection works across all Apps. Recent updates to 1Blocker has introduced additional network extensions, extending protection to other Apps.

A further to improve protection from exploits is to use a security focussed DNS Service in preference to automatic DNS settings. This can either be set on a per-device basis in Settings, or can be set-up on your home Router - and in so doing extends the benefit of this specific protection to other devices on your local network. I suggest using one of the following DNS services - for which IPv4 and IPv6 server addresses are listed:

Quad9 (recommended)

9.9.9.9

149.112.112.112

2620:fe::fe

2620:fe::9

OpenDNS

208.67.222.222

208.67.220.220

2620:119:35::35

2620:119:53::53

Cloudflare

1.1.1.1

1.0.0.1

2606:4700:4700::1111

2606:4700:4700::1001

Use of the above DNS services will help to shield you from “known bad” websites and URLs - and when used alongside 1Blocker, or other reputable Content Blocker, provides defence in depth.

There are advanced techniques to further “harden” iOS/iPadOS (such as using DoH, DoT and DNSSEC); while fully and effectively supported by iOS/iPadOS, Apple doesn’t expose this capability via device settings - but there are easy ways to access this functionality; a really easy way to set and manage DoH/DoT settings is to use a third-party utility App - DNSecure:

https://apps.apple.com/app/dnsecure/id1533413232

This App does exactly what is needed to effectively configure DoH/DoT - and is free to download and install. Many DNS providers are already preconfigured - including Quad9 and Cloudflare. Additional secure DNS providers can be added if required.

Apple has recently introduced its new Private Relay to its iCloud+ subscribers - in part employing ODoH (a variant of DoH) as an element of this new functionality. More details of this feature can be found here:

About iCloud Private Relay - Apple Support

In summary, there are many mitigations that you can use to better secure your iPad from malware and other potential threats. A good content blocker, combined with secure DNS and other protections offered by iPadOS can reduce your exposure to malicious links, content and threat actors.

I hope you find this information and insight to be helpful.

Okay. That's different. It's a scam web page that doubles down on the fear factor by also throwing a pop-up in for good measure.

It's meaningless garbage. All you need to do is close the page and avoid that site in the future.

If these warnings are sliding in from the top right of the screen, a site you visited has given itself permission to send push notifications. Everything you're seeing is nothing but a scam.

Open Safari's preferences. Click on the Websites tab and then scroll down to Notifications. Clear any entries in the right hand window. You might find something similar in the Pop-up Windows heading below that. Block any you don't recognize.

Then if you want, uncheck the box below as shown here.

I personally can't think of any reason why I would ever want any website to push notices to me.