My ceiling fan is in a catherdral ceiling and I cannot access it. Does the receiver have to be changed or just the remote. I've had it for five years and it is just recently that the problem has occurred that the lights come on by themselves. I have changed my TV remote which is DirectTV but the TV is not on at the time.

It MAY be a neighbors Light/Fan Remote. I would change from default code. Then Change your remote to match the fan code.

Yes, you will need to be able to access the fan, to change the frequency of the receiver as well as the remote. See:

Ceiling fan remote controls - help and FAQ- Ceiling Fans N More