First of all make sure you park the tractor at a safe place. The place can be your garage or any part of a road extension, where there is no vehicular and human traffic. The place especially should be free of children presence.
Have all the necessary equipment ready. For example, you will need a bead breaking tool. So have it with you and some other tools as well. Be assured you are carrying out removal of tractor's big tire, and if you do not know how to do it yourself, it is always better to have the job done professionally from the garages, where they also have purpose build machines.
You will need a pneumatic bead breaking tool. Make sure you follow manufacturer's instructions while using all the equipment. Do not make any adjustment on your own, as it cannot let you carry out your work successfully and quickly.
Next step is the breaking of the bead. Bead refers to the task how the tires is set on the rim and you will need to break it to remove the tire. Breaking of the bead takes special skills and technique. Nevertheless, by exerting force on the side of the tire you can break the bead open and then remove the tire. For this part of the job you will need a derimmer. So use it while carrying out the task. A simple way of breaking the bead is exerting of force on one side of the tire. However, you should be careful while exerting the force and never let lose your balance. It can be very risky.
At last, actuate cylinder on the rime derimmer to remove the rim. This makes your job almost done but make sure you inspect the rim soon after removal. This is to see if the rim has been damaged while the tire was being removed, and if it has, you will need to get it fixed before reusing. Now, you can safely replace the tire of the tractor, and the execution of work is almost similar to the removal of tire. The work is very hectic, so take your time for its completion.