If you have found an old bank note (dollar bill) tucked in an old coffee can or somehow received one in circulation, there are a few ways to change it. What you do will depend on whether the bank note is simply worn, torn or damaged, or is an older note that is no longer in circulation.
Look at the bank note and determine the year it was printed. The gold standard dollar was created in 1928. All dollars from this period on may still be in circulation. Determine if your bank note is old or merely in bad condition.
Take your bank note to your bank. If the bank note is worn, torn or damaged, you can exchange it for the dollar value. Keep in mind that you can't exchange a bill that is torn in half if you only have half the bill. You need both halves to exchange the money. If the bank note is an old note that is no longer in circulation, they may exchange it, but only after it is checked and authorised. Because out of circulation notes are not common, experts must determine if it is a forgery or not.
Contact a currency collection organisation, such as allcoins.org. These organisations have access to professional coin and currency collectors, and may be able to tell you if your bank note is worth more than the face value. Make sure you are able to provide them with all pertinent information, such as year of issue, serial numbers, face value and description. You may even want to scan it and send them a digital file. Your bank note may be worth much more than you imagine. Keep in mind that most bank notes more than 100 years old are worth more than the face value.
If you suspect your bank note is worth more than the amount printed on the bill, do as much research as you can to make sure you are able to exchange it for an accurate value. It is easy for someone to take advantage of you and profit on your naiveties. Be one step ahead.