How can someone erase cassettes that already have programs on them?
Cassettes can be easily erased. The simplest way is to just record something new on top of the old information. Alternatively, if you want to erase a cassette entirely, you could put a computer cassette into an ordinary music tape player and, after turning the volume down, press the play and record buttons simultaneously. Another way to do it is to use a bulk eraser, a device that uses a strong magnetic field to erase tape. These devices are available at record and electronics stores.