Empty all your files from the cabinet. Even though most file cabinet drawers are able to support dozens or even hundreds of pounds of weight, they are designed to be housed within the cabinet. Moving a fully loaded drawer outside of the cabinet will put a strain on the drawer, possibly bending or breaking the unit. Additionally, it's easier on you, since an empty drawer is obviously lighter.
Slide the drawer out as far as it can go. Try gently lifting the drawer upward. Some light-duty file cabinet drawers do not lock into place. To release drawers from a light-duty unit, you'll simply need to lift the drawer over a small "catch" or bump on the track. After lifting the drawer over the catch, it should slide right out. The drawer will typically only lift an inch or two over the catch. If the drawer does not lift up and out, you'll need to release the drawer locking mechanism as indicated in Steps 3 and 4.
Push the small metallic or plastic "release tabs" on each side of the drawer's track. The release tabs will only be visible when the drawer is kept pulled out as far as it goes. The release tabs act as a second catch to stop the drawer from sliding out involuntarily. On some filing cabinets, such locking mechanisms are released by pushing the tabs up or down, but in most cases you'll simply push the tab inward as indicated by NationalOfficeFurniture.com's guide to removing lateral file drawers.
Pull the drawer out while continuing to hold the release tabs. When the drawer is successfully removed, you can let go of the release tabs, letting them return to their normal "locked" position.