You will need to know how to adjust your Shimano Acera gears whether you are climbing mountains or commuting to work. If your bike has been over-shifting or under-shifting, consider adjusting your gears before your next ride. Some adjustments will require the use of a few tools, and minor adjustments can be made without any tools at all.
Put your bicycle on a rack, or hang it by its handlebars and seat. Find the Acera derailleur system near the rear wheel and gears. Locate the three Philips-head screws; these are the limit adjustment screws. There will be letters indented in the metal near these screws; the letters are B, L, and H.
Shift your bicycle into the lowest gear using the Shimano shifter on the right-hand side of the handlebars. Your chain should be on the biggest gear sprocket. Look at the chain and see whether the guide pulley is just clearing the large gear sprocket when you peddle the bike with your hand. If it is too loose, use a Phillips-head screwdriver to turn the B-tension screw one quarter-turn to the left. Keep adjusting until the guide pulley just clears the large gear sprocket. If the guide pulley and large gear sprocket are too close together, turn the B-tension screw clockwise one quarter-turn until adjusted.
Shift the bike into the highest gear using the Shimano shifter on the right-hand side of the handlebar assembly. The chain should be on the smallest gear sprocket. Find the cable barrel adjuster; it is the bolt below the H and L Phillips-head screws.
Using your fingers, turn the bolt clockwise one complete turn to give the cable some slack. Inspect the chain. If the chain is perfectly centered on the small gear sprocket, retighten the bolt. If the chain is near the next gear sprocket, use a Phillips-head screwdriver to turn the H screw counterclockwise using quarter-turns until it is centered. If the chain is too close to the bicycle axle, use your Philips-head screwdriver to turn the H screw clockwise one quarter-turn until centered. Tighten the cable barrel by turning it counterclockwise.
Shift your bike back into the lowest gear. Your chain should be on the biggest gear sprocket. Pedal the bike with your hand. Watch the chain on the big gear sprocket. If the chain looks as if it is tilting toward the spokes of the wheel, use your screwdriver to turn the L screw using quarter-turns to the right until it is centered. If the chain looks like it wants to skip the gear, turn the L screw to the left using quarter-turns until the chain is centered.