I have imac late 2015 should I upgrade Catalina 10.15.7

Yes.

I can't think of any reason to stay on Catalina over Big Sur if your iMac is supported by both, and yours is.

You may even wish to upgrade to Monterey, which is the newest version of MacOS to support the late 2015 iMac.

How to download and install macOS - Apple Support

Of course, there is always a tiny risk to upgrading so make sure your backup is up to date. If you don't have a backup this is a good time to make a time machine backup drive.

My late 2015 21.5" iMac runs beautifully on Monterey. Why upgrade to such an old OS?