Mirrored are pretty much the same as other window tint films.
They come off with heat. Heat gun will work, but best trick I've found is a steamer. We have a bigger unit, but started out with a handheld unit from one of the big-box stores, they're reasonably priced, and handy for a lot of things besides this. They have plenty enough heat to release the adhesive, and won't shatter glass, break seals on thermo-pane glass, or warp/ damage/ melt anything else by overheating like it's so easy to do with a heat gun. How easy or clean it removes depends on age & quality of the film, just like vinyls. They usually come off pretty clean for me, if it doesn't, just use a good adhesive remover or goo-gone for residue. If it's really stubborn, last resort is to scrape it off with a blade.
This is the same method I use to remove tint films. If it's on exterior windows, don't do it on a cold day, it'll take forever.