Where can I see the available updates on my iPhone 13?

You’re welcome! The next major release will be iOS 16, which is expected in the Fall. While there is a public beta currently available, I do not recommend installing a beta, as the whole point of a beta release is to find bugs. Apple recommends against installing a beta on a device that you must use every day, both because you may discover a major bug, and also because you can’t get support for any problems you have with a beta release.

Go to Settings/General/Software Update and it will either tell you that you are up to date, or offer to update your phone.

The current iOS version for your phone is 15.6.