By 'replace media' do you mean swap one image for another or something??
If so, all you need to do is select the image(s) you wish to delete and hit the delete key; then add the new image(s) Files (file/add/existing files -or- the green circle with a white plus sign in the middle on the toolbar).
I've only used the Windows version and think it looks awesome! AFAIR the only differences in OS versions is that Linux does themes better...but I'm not sure that is a Sigil thing??
Can you post a screen shot of each to show the difference? If it is WAY better I might have to request my $$ back!
BTW - Unfortunately the Sigil pros don't accept donations, they do all this for free. If you see a donate button on a webpage somewhere it is NOT going to the current developers. I think they mentioned it (the donate button in Sigil) may still go to the original developer from long, long ago...