my son has a vertigo nordend bike and the gears keep on moving whilst cycling and the chain always slips in between the gears and when chain gets on the gears it just spins,not sure how to fix this hope someone can help
Your description of the problem is not to clear. From what you describe it may be a freewheel issue. Or this could be as simple as a gear adjustment. More detail would be helpful.
take it to a shop
The answer would have to include a complete explanation of how to adjust a rear derailleur and maybe how to replace an entire cassette depending on what is going on, which isn't clear. Take it to a competent mechanic or buy a good book on bicycle repair and learn it all yourself because the question, in my humble opinion, can't be answered in any reasonable manner in a forum.
Originally Posted by hethyrocks