Here are couple of things you need to do:-
1. Sit at the edge if the bed. Slowly bend your knee and straight it. If its painful take assistance while doing.
2. Apply ice packs for 10 minutes on the knee and do the knee exercises.
3. Lyedown on the bed and Place a writing plank or smooth plywood plank underneath your leg and try to bend your knee slowly as its friction- free bending will be easy .if required take assistance.
4.knee cpm machine may help you in your knee bending.
5.consult a orthopedic Physio for intensive knee rehab.
It is difficult to bend knee post knee replacement. It is vital to get physio sessions done regularly till 100-110 degree range of motion is achieved by the patient actively! With correct physiotherapy patient will be able to achieve the desired bending for sure.