Replacing the lens on a Canon DSLR is a simple process. You may refer to the image below for a visual. 

Step 1: With Lens-release button pressed in, rotate lens assembly counter-clockwise. Lens will come off

Step 2: Insert replacement lens while lining up white mounting index square on replacement lens with white mounting index square on camera body. Rotate lens clockwise until you feel it click into place. 

Place the lens in the lens mount with the mounting indexes aligned.