How can you reverse the process of contraction in a stroke patient?

Two aspects. There are two aspects to a contracture in someone with a stroke. One is spasticity which is an overactivity of muscle contraction. This can be dealt with quite effectively with injections of botulinum toxin. The second aspect is the contracture of the tendons and joints if the contracture has been long standing. If this is a predominant issue, techniques such as serial casting or even surgery .
Rehab/Therapy. Physical or occupational therapists can work with stroke patients to help minimize contractures. The sooner you start rehab/therapy after a stroke, the more successful your outcome will be.