I can't update since iOS 13.3

There’s a good chance the phone had an iOS 13 beta on it. You will have to remove that to update it→How to uninstall iOS beta software - Apple Support

Other than that, update over the air from the phone all of these requirements must be met:

• You need at least 6 GB of free storage
• You must be connected to a Wi-Fi network (and not a personal hotspot)
• The phone must be charged to at least 50%, and preferably plugged in so it stays above 50% during the update
• The phone must not be in Low Power Mode
• The phone must not be connected through VPN, and it may be necessary to delete any VPN apps and VPN profiles
• The phone cannot be in Airplane mode
• You cannot still have a beta version installed on the phone
• You cannot have a previous version that was downloaded, but not installed - go to Settings/General/iPhone Storage to look; if you find an iOS installer, delete it

If you can't meet all of them you can still update using your computer.

Florida-SunshineWoman wrote:

Ok. I thought your company was a branch of Apple. Thanx anyway!

This isn’t a company; it is a user-to-user technical support forum that is hosted by Apple, but Apple does not participate directly.

Like just about every large company Apple has both formal technical support and an area where users can discuss technical issues with other users. And, like all of those other companies, Apple does not participate in any significant way in the user-to-user discussions, as that might inhibit the free flow of ideas. The formal name of this area is Apple Support Communities. The “Communities” should be a giveaway that this isn’t a formal support site. And if that doesn’t, the site itself is https://discussions.apple.com. The “discussions” is another giveaway.

Apple’s formal support can be reached several ways:

• Click on “Get Support” at the top of this page
• Go to Apple Support

Note that both of the above have options to receive a callback or chat

• Install the Apple Support app on your iPhone or iPad - you can contact support directly from the app, and the tech will have (with your permission) to diagnostic data on your phone
• Call Apple Support; the phone numbers are in this link→Contact Apple for support and service - Apple Support
• Make a Genius Bar appointment at an Apple Store→Genius Bar Reservation and Apple Support Options - Apple
• Find an Apple store or Apple partner service provider→https://apple.com/retail
• DM @AppleSupport on Twitter
• You can also provide feedback to Apple at→ https://apple.com/feedback

About the only communications method not used by Apple support is inbound email, probably because the volume would be overwhelming, and separating the wheat from the chaff (or, to us engineers, a poor signal to noise ratio) would be almost impossible. 

This is a user forum. Everyone here is a customer just like you, volunteering their time to help other users. Apple is not here.

If you want a response from Apple support, then contact Apple support.

In addition, it should be noted that there is no such thing as an “iPhone 6SE”.

At this stage, I recommend you make an appointment at the Genius Bar of your local Apple Store.

On what iPhone Model? Where did you buy it? Was it a brand new device, or did you buy it second-hand?

What happens when you try to update it? what error message(s) are you getting?

How are you trying to update it, wirelessly or via computer?