americanlt2

Whenever I stand up to pedal or put a lot of pressure on them when I peddle the chain jumps to a higher gear (making it harder to pedal). Seems kinda dangerous & is annoying. Any idea on how to adjust the gears in the back wheel?
Thanx

kv501

Read this front to back (particularly derailleur adjustment in your case).

Also, if your chain is skipping check to make sure there is less than 1/16" wear in 12 inches, and replace the chain if there is more than that. If it still skips after adjusting the derailleur and replacing the chain buy a new cassette/freewheel.

If you are confused after reading the Park Tools information hit up Youtube; there are some good videos on bicycle maintenance topics.

JohnDThompson 

+1 worn chain. Be aware that if you replace the chain you will also likely want to replace the cogs/freewheel.

FBinNY 

I agree that slippage under load is usually an indicator of worn chains. But in your case it may not be because that type of slippage usually doesn't also cause an upshift. Start by measuring your chain for wear (stretch) to rule it in or out as a factor (slippage rarely occurs in chains stretched less than 1/8").

Then check the RD trim adjustment to make sure the RD is centered under each sprocket. If you're unsure, try turning the cable (trim) adjuster outward by 1/2 turn. If that improves, or at least doesn't worsen shifting, it might improve your problem. If the slippage situation improves but doesn't resolve completely, try another 1/2 turn. In fact you can do this by degrees until the RD doesn't upshift crisply, then back off the last 1/2 turn and leave it there.

If you're lucky, your chain is OK and the trim correction solves your problem, but measure the chain anyway, because you want to replace it when it stretches past 1/16" so you won't have to also replace the cassette.