I am taking prozac 40mg since a few years. my doctor has asked me to taper off by shifting to 20mg immediately. is that fine and without side effects?

You may or may not: experience Serotonin Discontinuation Syndrome.

Give it a try: If your doctor has advised you to reduce the dose, that is a reasonable suggestion. You may not need the higher dose if you do not have the symptoms for which the medication was prescribed. I would recommend that you exercise regularly. Exercise is an effective treatment for mild depression. Wish you good health.