Use a heat gun. Turn the heat to a moderate setting and increase it only if you find it absolutely necessary. Heat a small section of putty, and gradually dig out the softened putty with a 1-inch putty knife.
Use a hand-held electric soldering iron with a 40-watt screw-in tip. Insert the heated iron into the putty. Then use the iron itself to dig out the softened putty.
Try a hair dryer on the highest setting if you don’t have either of the other tools. The hair dryer may not be hot enough for the job, however, and you might have to buy, rent or borrow a heat gun or soldering iron.