Hi Paul.
Yes they are easy to change,and actually quite easy to clean and service!
Are you sure its the lock barrel mechanism and lever thats stiff, "due to dirt,loose screw",and not another part of the mechanism?
1st try sticking a WD40 tube in it and drench it,then wiggle it with the key,and flush with more WD40,that maybe all it needs to clean it out!
Otherwise,you need to pop the inner door panel off and investigate!
Remove it and soak it in petrol overnight to loosen any old polish thats got into it,then a good squirt of WD40 and operate it to get things moving and loosen any dirt,then flush with more WD40!!
