Help with exchange box a BMW M5 of 1984

I want to know if the transmission of a BMW M5 can be disassembled and what steps need to be taken to do so.
I was told that the gearboxes are sealed but that they can still be opened since the reverse gear is stuck because it broke and I want to replace it.

All gearboxes can be disassembled, because they're assembled before being put back together if it's a manual transmission. There's little to take care of; mainly, double-check the position of the gear selector's synchromesh so that once you reassemble it, you don't find yourself missing a gear or without reverse. But as a tip, disconnect the gear selector from the lever to the transmission and test if the gears engage and disengage manually. Sometimes the selector gets misaligned or moved, and that's the whole problem. Do this before disassembling it unnecessarily. Good luck