As a shop owner/business owner you should contact your local crime prevention officer who will be able to advise you on preventing further instances of this happening, and can ascertain whether there are any other shops or businesses in the area that has received any so that appropriate action can be taken. Also see the website in Related Information for more details.
If you are a customer and have been passed a forged bank note in your change, hand the forged bank note into the local police station and give the details, if at all possible, of where you were given it. The police will keep the note as evidence.
UNDER NO CIRCUMSTANCES try to use or pass on a forged bank note to someone else, the penalty could be up to 10 years in prison and it just isn't worth the risk.