It can be done but will be expensive as you need to replace the full drive train. 7 speed is wider so new rear wheel or hub, 6/7 speed index derailleur, 6/7 speed index free wheel and 6/7 speed shifter. I changed 2 Dahon Board Walk bikes like this from 6 to 7 and it ended up costing about $50+ per bike. In my case the replacement was from 6 to 7 and the reason was the 6 speed stuff was worn out and very hard to find. The 7 speed parts were actually cheaper. Roger
Last edited by rhenning; 05-13-19 at 06:40 AM.