Why can’t I change my stand or exercise goals?

For some reason (with watchOS 5 and 6?), you can change the Move goal, but not your Exercise or Stand goals.  I believe this was changed in WatchOS 7 so that you can now change all the goals. See the "Change your activity goals" section in:

Use the Activity app on your Apple Watch - Apple Support

What version of watchOS and iOS do you have? For some combinations of the older watch model and iPhone you may not be able to update to watchOS7.

If you have WatchOS6 then this is all you will see;