Shimano Acer shifters, which are also known as a derailleur, is the main component used in mountain bikes for shifting from one gear to another. Eventually all shifters need to be adjusted to prevent the chain slipping and shifting speed in perfect. With patience and some free time, you can fix them yourself rather than visiting a bike shop.
Clean all dirt and mud from the cabling and ensure they are not being caught on anything, making sure that the shifting problems are not a result of a warped, dirty or stretched chain, clogged up pulleys or cables.
Position the bike on a bike stand. Using the rear gears, move the chain onto the highest gear (the smallest cog) with the rear shifter levers. If there is slack in the cable, loosen the cable bolt on the derailleur using a screwdriver and pull the cable through until it is tight. Retighten the bolt to hold it in place.
Locate the high and low limiting screws on the derailleur. Loosen the high gear screw, with the chain still on the smallest cog, adjusting the pulley in the derailleur until it is exactly in line with the smallest cog. Once they match, tighten the screw about 1/4 to 1/8 moving it slightly closer to the other cogs.
Shift into the lowest gear (the biggest cog) with the front derailleur in the in the highest gear (smallest cog). If the gears won't go all the way, loosen the low gear screw until it does. Adjust the derailleur until it matches the teeth of the large cog. Tighten the limiting screw until it moves slightly toward the other cogs.
Shift back down to the highest gear and begin moving from one gear to the next ensuring each time that a cog is not skipped. If it doesn't shift down gear, tighten the high gear screw until it will shift.
Repeat the processes for each cog until you are satisfied that, with each gear change, the chain is moving to the next cog without slipping. Ride around on the bike to make sure you are completely happy with the gears changes. If you are not happy, repeat the process again.
If you do not feel comfortable attempting to adjust the gears, take the bike to a qualified bike shop who will be able to set the gears up for you for a price.