Small amounts of exercise can help increase your strength, flexibility as well as improve mobility and strengthen the heart and lungs, and of course help control or reduce weight.
You don't have to go to extremes with exercising like these bodybuilders with disabilities, however there is no reason why you can't if you want to.
Before any exercise session, it's always a good idea to warm up, stretch and cool down for about ten minutes before and after. Always start slowly and work up to more challenging exercises, don't go crazy. An upper body workout would include exercises for the arms, torso, neck, and shoulders. It all helps.
If you are concerned its always best to consult with your doctor first to find which exercises are best for your needs.
One type of exercise you might do from a wheelchair is so called resistance training.
It uses large, stretchy rubber bands called resistance bands. Wrap the bands securely around a door, pole, arm of your wheelchair, or other stable object. (make sure it is stable first) then pull the bands towards you or away from you to work your muscles. You can use the bands to do arm and leg extensions, shoulder rotations and pull downs if you are able to.
Another type of wheelchair exercise is strength training.
This is done by lifting small weights or dumbbells. If you don't have weights or dumbbells try cans of food or bags of sugar etc. Look around. Weigh them if you can. Start with 1-2 pound weights and work up slowly.
The benefits of strength training include the ability to perform daily activities, like pushing wheelchairs, carrying items and transferring. Many wheelchair users have an inefficient 'push' and strengthening the muscles involved will greatly help.
Remember to always keep a positive attitude. Use discipline to get into a regular exercise and fitness program you've created for yourself and continue to do it. It is so easy to give up but important to keep it up but remember don't do anything you know or feel you shouldn't. Start slowly. Keep your eye on the end result.
A good fitness and wheelchair exercise program can significantly improve your health and increase mobility for you as a disabled person. If you find yourself getting bored, stick some music on, it helps a great deal.