One of my internal door in the house got jammed recently and I decided to attempt the change by myself. Since it was just an internal door inside the house, I thought it was rather safe to give it a try (highly recommend a professional locksmith to the job if it is for the main door / entry to the house or if you feel the lack of confidence to do the job).
What you need:
A new set of door lock
Philip head screw driver
remove the old lock
1. Remove the old lock from the door. (Bring it along to the store if you're unsure of the lock size to buy for your new door)
2. Insert the latch into the holes on the edge of the door.
Tighten the screws
Secure the faceplate by tightening the screws provided in the lock set with appropriate screwdriver.
3. Install the front lever assemblies to the door by inserting the spindle of exterior handle through the latch cam.
4. Proceed to install the back lever assemblies part by part. Fasten the under-rose first, then followed by the rose and back handle.
(In the event when interior and exterior handle installation directions are conflicting to the default setting, you can exchange the handles to suit your door. Follow the manufacturer's installation manual for special instructions, if any)
Fasten the strike plate with screws
5. Finally, install the strike plate on the door frame by inserting the strike box first then the strike plate.
Now it's time to try out the lock and make sure it is working alright.
Do note that different locks may require slightly different installation steps but in general, these are the common steps to change a basic door lock.