Some have used hair dryers to soften the old glue.
I've (very carefully) used solvent. Just be careful not to let it get on the cloth / stitching
Wash with soap and water afterwards.
And if you ever think about putting some new stickers on...............
Cut off an inch or so of the backing all the way round and then stick on. 90% of the adhesive is then not in conatct with the sail.