Can anyone tell me how I can go about replacing the brake lever?

Thanks in advance.

Unscrew the old one from the master cylinder using a flat head screw driver. Put the new one in and put the screw back in.

Don't forget about the 10mm nut underneath the bolt.

Just in case you don't do a lot of wrenching, here is a general rule of thumb:

Use a suitable wrench (one that fits properly) to remove the NUT; use the screwdriver (or whatever tool might apply in a different situation) to HOLD the other end and keep it from moving. IOW, do NOT use the method Alpha suggested; turning the screw will be difficult with the nut on the bottom. Use a 10mm wrench (or an adjustable wrench, or whatever else you have available) to TURN THE NUT; the screwdriver on the other end is simply used to keep the screw from turning.

Think about that for a second; if you have a bolt (or screw) holding something together, the nut is the fastening / tensioning device, while the head of the bolt is on the opposite side of the component (whatever it might be). A few weeks ago, I watched one of the local village idiots (who INSISTED that he knew what he was doing) snap the head off the bolt through the top of his shock absorber; he held the nut with a wrench, and turned the bolt head with a socket on a breaker bar... within a few seconds, he'd snapped the head off the bolt and started cussing up a storm. He SHOULD have held the head of the bolt with the wrench, and used the breaker bar to turn the nut, but some people just have to learn the hard way...

Hope this clarifies things for you; if you aren't sure how to remove or install a fastener, simply remember that you want to apply the force with the tool to the "working" end of the fastener, which is the end where the threads are located, and hold the other end of the fastener (the "stationary" end) to keep the bolt from turning. That simply means that to remove / replace your brake lever, the "working" end of the screw is where the nut is, and the "stationary" end is where the flat-blade screwdriver is used. Turn the fastener on the working end, and hold the stationary end to keep the bolt from spinning.

Lefty-loosey, righty-tightey