You can unzip it and edit, but it’s not really a good idea to do so. If you are changing a third party module then you are locking yourself out of future upgrades and you could break dependencies other modules have on it.
If you do change it, I would strongly suggest renaming it so avoid clashes.
Alternatively, see if the source is up on GitHub. The author may be open to accepting change requests to the official module.
MPK = Mendix Package, which can contain a full Mendix project or a module. You can use this to use or to share functionality between projects.
Please use an external module carefully. Don’t change the original module, so you can still get updates. If you want to replace or to extend, please create your own module which uses your downloaded module as black box and build on top of it.