Gently lift the grill frame by one corner to release the Velcro, which will reveal the front-mounted speaker. From there, unscrew the speaker. Watch out for the speaker terminals as you remove it. As soon as you can reach the terminals, gently wiggle them off with small pliers if necessary. Note the + and - terminals and refit the same way on the new speaker, lower into place. Start the screws by hand to make sure they have entered the threaded T-nuts, and carefully screw in without letting the screwdriver poke the cone. Tighten moderately firmly but don't warp the frame.
Need some extra guidance? This video explains the process.
NOTE: Make sure your replacement speaker matches the hole opening and mounting points. Most steel frame speakers will match, but most cast aluminum frames will not match.
ALSO NOTE: If you have the discontinued MicroPro Mach 2, the process is a bit different. Info on how to do so can be found here.