If you want to do it right....remove the insert, sand until clean and flat.
I guess two options at that point....if the decal covers the insert only, you could install the decal on the insert when it is removed from the playfield, and spray a coat of clear over it and then reinstall it. (not the best option, as you would have lines between the clear on the insert and the clear on your playfield).
Best option would be to reinstall the insert and new decal, and spot clear over the entire area, feathering into the existing clear. If the decal extends beyond the insert area, I would sand those areas down flat and prep with a coat of clear prior to installation of the decal.
If you just scrape off the existing clearcoat and screen print and stick the new decal on, you most likely won't be happy with the results.
Maybe one of the pros can chime in, as I am definately an amateur. I think I know enough to know that doing even a single insert decal is a major job if you want it to approximate the look of the original playfield. Any shortcuts will probably look like you slapped a decal on your playfield, if you know what I mean. Good luck.