Remove the action from the stock. On the underside, at the opposite end from the trigger, is the anti-beartrap catch. Whack it gently with a hammer and it will release the piston. When you do this make sure the sidelever is fully closed and engaged on the catch so nothing can get trapped. It would also be prudent to open the loading tap to provide an air cushion to slow the piston.
If you are really going to take it apart, I am slightly concerned that you do not know how to do this, as Webley sidelevers can be dangerous and bite very hard - trust me, I know from experience. But if you do, there's nothing particularly complicated, but I would offer you some tips.
1. The cross pin holding the end block in place is also secured by a hidden allen grub screw. It's down a hole at the left side of the cocking lever hinge. This is there because the sideways torque of the cocking lever can make the pin move across to the left. Don't forget to refit it.
2. If a Tracker has a normal spring fitted, you can get away without a compressor. Use a parallel punch of appropriate size to drive out the pin - the punch then holds the block in place. Then force the rear of the gun against a hard surface and maintain that pressure as you wiggle out the punch. Sometimes the block even sticks in place, and needs a light tap with a hammer to pop out. If it's been fitted with some type of magnum spring, however, none of this applies.
3. Refitting the cocking lever can be very difficult if you don't know how. Before fitting it, the anti-beartrap mechanism must be fitted and working, but you can leave the trigger assembly till last. Then, you fit the cocking lever without the spring and just a parallel punch or screwdriver as it's pivot pin. Now cock it just three clicks on the anti-beartrap. This moves the piston back just enough so there is no pre-tension on the lever, and you should now be able to fit the pin and spring without much trouble. I do this with the muzzle resting on the floor and the gun vertical, this keeps your hands away from the anti-beartrap release. Work in such a way that your fingers are never under the lever, and again, open the loading tap. The cocking lever pivot pin has a very small circlip, which you must refit if you didn't lose it on disassembly!
4. This applies to many Webleys. If you manage to replace the safety catch and spring, well and good. If you find it a complete faff and struggle with it, ditch the safety catch and replace the spring with a straight ended one from a Victor or Excel.