Remove protective cover, cushions and pillows from the couch. With the aid of a helper, turn the couch over and set it upside down on the floor. Measure the length and width of the legs. Remove one of the legs, if possible, and take it with you to the store.
Purchase replacement legs that are longer than the existing ones, but are the same diameter. Make sure the replacements install the same way, i.e., screw off/on or attach to the underside of the couch with a bracket. Install the longer legs and set the couch back up.
Raise the legs on a couch with furniture risers. Risers are typically available in heights from 2 to 5 inches. To reduce the chances of the leg slipping off the riser, the end of the leg should fit in the recess in the top of the riser.