Depends on what you’re finding hard to remove. If it’s hard water stains, do what
@StressCraxx said and use some vinegar or a dedicated
hard water stain remover to dissolve the calcium. If it’s sticky grease or rubber marks, spray on some
Mothers R3 and leave it on for a minute, then wipe it down with car wash solution.
If your decals are damaged, you can buy
replacement ones. You can ask a vinyl shop to help you remove your old one safely and apply the new one if you are afraid of marring the paint.