I would try Acetone. It isnt as dangerous as other thinners like lacquer thinner or Methyl Ethyl Ketone (M.E.K.) M.E.K. is immediately dangerous for your liver when absorbed through your skin.
Use Acetone, thats what is in nail polish remover with out the lemony smell.
Failing that use tape to form a square around the letter and paint a black back ground over the error and paint the letter on top of that.