To replace the clutch lever, you need to remove the single nut from the bottom of the headset on the clutch lever side. Then, using a flat-head screwdriver, unscrew the threaded bolt from the bottom of the headset. You now have to disconnect the cable from the lever. You will need to get down by the enigne and loosen the pinch bolt so you can slide enough cable up to give you spare cable at the lever to disconnect the cable from the lever. Then, be careful as you remove the lever as there are two small washers that will come out with it - one on top and one on the bottom. Save these, as you'll need them!
This is probably also a good time to replace the cable. Inspect the cable outer to make sure it's in good shape. Lubricate the new clutch cable using a lube of your preference - grease or chainsaw chain oil etc (each has proponents and detractors). Thread in your new clutch cable from the headset. Don't connect the cable to anything just yet.
Now install your new clutch lever. It can be tricky with those little washers, but be persistent and you'll get it. Grease up the clutch lever on both sides and in the bolt hole. Put a washer on each side of the clutch lever and insert the threaded bolt through the bottom of the headset, through the washers and clutch lever, and into the threaded part of the headset. Be very careful with threading the bolt in because it is super easy to strip the threads in the headset (which is a difficult repair). Once you've got the bolt threaded in correctly, thread on the nut and tighten it to the point where it is nice and snug but you can still operate the clutch lever with very little resistance.
Now put the clutch cable into the retaining hole on the clutch lever. Grease this area well as it sees a lot of movement which causes parts to rub against each other and wear. Pull on the clutch lever from the engine side of the frame and make sure things are working well. Thread the cable properly through the adjuster and clutch actuator arm on the engine side. Attach the pinch bolt behind the clutch actuator arm and adjust the clutch cable to your preference.