Is it easy to change your gears to a lower or higher on a ss?

Do bikes like these have that option:

www.bikesdirect.com/products/dawes/sst.htm

http://www.bikesdirect.com/products/.../messenger.htm

I did talk to a doctor and he said I need to straighten my legs and this will help my knee. I talked to a physical therapist and he said riding a bike would be great.