How to get rid of “ unsecured network” on my iPhone?

That message indicates the Wi-Fi security is lacking.

If this is your network, you can enable Wi-Fi security on the network router. What can be enabled depends on the specific router and often also on its firmware. Update the router firmware ro current, when you access the router to reconfigure the Wi-Fi security setting. How to,do all of this varies by network router model and firmware.

If this is not your Wi-Fi network, you can ignore the message as nothing has changed, other than that you now know the network has little or no security. Depending on the details here, you might need to follow a few steps up to and possibly including a network-settings reset.

Details:

If your iPhone, iPad, or iPod touch won't connect to a Wi-Fi network - Apple Support

Recommended settings for Wi-Fi routers and access points - Apple Support

The way to remove a warning is to heed its advice.

When you're warned of an unsecured network, secure the network.