If the bulbs are hot, allow them to cool.
The cover may be held on by screws or clips, or it may simply lift and slide out.
Holding the tube with both hands, gently turn it one-quarter, and pull it straight down. If the fixture contains more than one bulb, consider replacing them all at the same time to simplify maintenance.
Link up the two prongs in the socket, gently push the tube upward and turn the tub one-quarter turn.
Replace the cover in the same manner as you removed it.