You will need to get hold of a depth gauge guide. Sizes vary so it must be the correct one for your particular make and model. You should be able to purchase one for your manufacturer, if one did not come wit your chainsaw (see Manufacturers)
1. Put the gauge guide on the top of the chain and you will see one gauge sticks out from the aperture. (see below)
2. If you see a gauge that is above than the guide, use a file or similar to file it down to the right level (lower than the cutter) Try not to take it too far down and try to file evenly so the gauge retains its shape and integrity, to keep a smooth run.
Perform this task across the whole chain, ensuring you file both sides for an even run.
This procedure will make the chainsaw cut more evenly and will prolong the life of your chain, saving time and money.