My Renault Clio car(used) has old,dirty & lax seat belts.Can I change them with new, without mechanic's hel

Sure you can , just go to a big do it yourself auto wreckers and take some off of their cars.It is a good idea to buy a Haynes service manual for your car it will help you a lot in the long run by the way.

Hope that helps and best of luck , by the way seat belts are not that hard to replace but only change one at a time so you've something to compare to.