Stop those bike gears slipping simple tightening of the gear cable with an anti clockwise turn of the barrel adjuster usually resolves most gear changing problems.
Mountain bike rear sprocket slipping.
If the chain is worn out replace it.
Skipping can also be cause by a worn chain worn chainrings and worn sprockets step 2 remove the rear wheel.
If a link is bent either replace the link or the entire chain.
Using a chain tool slip each of its ends through the links of your chain and check your chain against its wear indicators.
If not check cable housings chain and cogs for wear and see if the rear derailleur arm is bent.
I guess that the spring isn t hold the sprocket well.
But if you want to repair it all alone first get your rear wheel off get the sprocket off and check the 3 teeth inside the sprocket they have to be perfect half circles.
Disconnect the rear brake and undo the axle nuts step 3 remove the cassette.
To fix a skip in the rear derailleur shift your chain into the smallest ring on your rear cassette the hardest gear and the middle or larger ring on your front derailleur.
Step 1 make sure that the problem is caused by the freewheel.
The freewheel mechanism is contained inside the splined housing.