Can you change a Minecraft Mod yourself so that you can play it on 1.16.5 and if so, how?
If it is open source and the necessary skill is available, yes!
If you can program, you can do that without much effort.
Since you're asking the question, you probably can't do that, and neither can you update your mod to a new version.
(Even if that's not okay.
Java files (.jar) as mods they are (as far as I can remember) can be decompiled without problems and thus read the source.
With the ability I have to agree with you, without the know-how nothing works here.
As the others have already answered correctly - this is basically possible, provided you have the appropriate knowledge and access to the code.
Of course, it is important to know why the mod no longer works. I'm less familiar with Minecraft mods, but as an example: If the name of a function or a variable changes, it can of course be adapted very quickly.
However, if the mod requires features that have been removed from the game, repairs are more difficult, and in some cases even impossible. So it depends on the mod.
