The following exercises for knee injury should be done in a pain free range and are just a general treatment without assessing what is the cause of your pain. Please seek advice from a professional.
Standing in font of your couch place one leg behind you so that its bent and your shin is against the back rest of the couch. Your knee should be on the cushion and be as far back as you can get it. Then squeeze your buttock so that your glutes are nice and tight. Keeping them squeezed bring your hips forwards while leaning back a little so that you feel a stretch on the front of your hip and/or quads (front of your thigh). Hold for 1.5 – 2 mins the repeat on the other leg.
One of the best ways to help with knee pain is to strengthen the muscles around the knee. This is a great exercise for your knee muscles that help control the knee cap.
Place all of your weight on one leg. With your non weight bearing leg you want to reach it out so that you tap the floor with your heel as far in front of you as far as you can. This will mean your weight bearing leg will bend at the knee and you want the knee to bend as much as is pain free. Once you have touched the floor with your heel straighten your back knee and stand tall. Then repeat 10-15 times until you feel a nice burn in your quads. Repeat on the other leg.
This can be done on a wall or a door frame. I personally prefer the door frame as it enables you hold onto to something and also get deeper into the stretch. With a straight leg place one foot again the wall or door frame. You want as much of the underside of your foot on the wall/frame as you can rather than just the balls of your feet. Now gently bring your hips up and forward until you feel a stretch in the back of your leg (calf area). Now gently and slowly pulse your hips up and down so that you putting your calf on stretch then off stretch. Do this for 30 secs. Now bring your hips up so that you are in the stretch again. Keeping in the stretch push your hips out to one side and then the other side as far as you can. Keep this going for 30 secs. Now keep in the stretch and rotate to the left and then to the right as much as you can. Keep this going for 30 secs. Now repeat on the other leg.