As some have said, better quality cassettes are held together with screws, but some are glued (most likely a heat weld).

The ones with screws usually can be opened up to repair/replace the pads and then closed back up.

For ones that are glued/welded together, you'll have to split the case to get to any parts that need fixed. I haven't checked recently, but sometime back Radio Shack carried "repair cassettes" with no tape in them just for that purpose. The repair cassettes of course go together with screws. Other similar "audio/electronics" shops would be likely to have something similar, but it's getting hard even to find a good retailer for pieces and parts of that sort.

Even a lot of the S.... Shacks have quit carrying the DIY stuff, but you usually can order from their catalog.

John