I've given this subject some thought, as I have a "sticker" that is partially dislodged on my Mixte.

I have yet to try this, but I'm going to use some double sided (sticky) clear tape to refasten it. The tape I have is super thin (proly .003" - .006"), and I would assume work well with a decal too IF you can remove yours intact. A little X-ACTO or razor blade work in cutting the tape / decal outline should do it prior to re-affixing.

Thought about a tiny application of Super Glue (instead), but I think that might be too dicey and messy to achieve good results. There is a chance of destroying the decal (or sticker) or any surrounding paint work in the process.