Clamp the gearbox end joint in a vice with the driveshaft facing upwards (have an assistant to hold it there). You will notice three sheet metal tabs with their edges protruding slightly over the grooves that the tripod roller bearings slide in. These can be bent upwards by tapping with a chisel until the first roller race can pass over them. You have to tilt the driveshaft a little and remove one roller race at a time. Be careful, on many driveshafts the roller races are not held on the tripod joint by any means so you should be careful not to dislodge them, otherwise you will spend the rest of the day searching for bearing needles in the grease!
On reassembly, you simply have to fit the tripod joint in the cup - don't forget the spring- and tap the retaining tabs back in place.