What happens after module upgrade if you modify the default views I want to modify a view supplied by a module. What happens to the view when I upgrade the module? Does it overwrite my changes? If it doesn't and the views in the upgraded version of the module has changed, how do I get the new version?
It should not necessarily change your view, views shipped with modules are usually kept in code, if you change that view, you override it and have it in the database, as long as this overridden view is in your database and a module update does not drop these changes on purpose you should be good to go.

But, always do a backup and test on a local/ different environment than live.

If it does indeed change stuff, you will have to clone the current view and maybe even mimic custom views hooks that are added by that module for that view.

btw. it's worth reading the commit messages since the latest release and or changelogs, if you can read code it's worth looking into the module.install file, and looking at the module_update_n hooks, there you can see if anything major might get dropped as well.