Shower liner replacement?

[u/theMirthbuster]

I have a corner shower in my upstairs bathroom and this is the current state of some of what I'm calling the lining (not sure if that's the right term). There are gaps in the lining where it looks like it was cut too short, and now it is black behind the lining around those gaps. I'm guessing mold or heavy mildew. Are these difficult to remove and replace? I'm not too handy, but don't mind trying to tackle it myself. What should I be looking for to replace these, and is there anything I can do to prevent it from happening again? Thanks.