How to fix the chain on a bike?

To fix the chain on a bike, first, shift the gears to the smallest chainring and cog. Then, loosen the rear wheel and pull it back to create slack in the chain. Remove the broken link or fix the issue causing the chain to come off. Reconnect the chain and adjust the tension by moving the rear wheel back to its original position. Finally, tighten the rear wheel and test the bike to ensure the chain is working properly.