How To Remove A Decal From Your Mirror
Although I wouldn't recommend trying to remove decals from ornate/vintage/expensive mirrors as it does require some heat, for most mirrors it is super simple to remove your decal when you want to take it off.
Watch my quick tutorial here:
Start peeling off the letters slowly. You might be able to do it just like this, but if it's tricky to peel, get out a hairdryer.
Using a medium setting, move the hairdryer over the decal, being careful not to place it too close to the mirror or over the same spot for too long. This should make the glue tacky and it should be easier to peel.
You will still have to peel the letters off individually however so it can take a bit of patience, or you can use a scraper if you're not worried too much about damaging the mirror.
Once it is removed, it may leave residue. If so, use a glass or mirror cleaner and a sponge or cloth to scrub the residue, and then buff with a dry cloth or tea towel.
Your mirror should look shiny and as good as new!
If you want to know how to apply a mirror decal, visit this post here: How To Apply A Vinyl Decal To A Mirror