You need a whole rear wheel, new cassette and shifter. Going 11 speed you may have trouble with the chain riding on the chainring depending on how wide the chainring is. But you have no front shifting to worry about so it's easy. I would suspect you want to stay 10sp if the rear end measures 130 for best wheel strength. But 6sp is a screw on freewheel while 8,9,&10 are cassettes in which the mechanism is built into the hub. All three speeds use the same hub while the best you might do is a 7sp freewheel with the current hub.