The MPG on my 99 Ranger 4.0L is down and an engine check shows a lean air mixture. Mechanic thinks the oxygen sensors needs to be replaced. Is this a DIY for a non-mechanic person. How to?

 

This should be an very easy fix. It would be tricky to know which oxygen sensor it is as I believe your truck has 4 in the exhaust. You might be able to test them with a voltmeter or I would recommend replacing all since one went out there is a chance of others going out. Check out youtube. There are tons of videos on replacing them

 

On a V engine there are two O2 sensors that could effect MPG (lean/rich)
The "upstream" O2 sensors
These are nearest the exhaust manifolds, one on each pipe

Both should be changed, they usually unscrew easily, the PITA part can be the connectors, lol, I have big fingers.

Expected upstream O2 sensor life is 80-100k, they use a chemical reaction to measure Oxygen, so do wear out, engine problems can shorten their life as well.

The other O2 sensor, single exhaust, or two O2 sensors, dual exhaust, are called "downstream" O2 sensors, these are "after" the Cat Converters, they are there to check the Cat converters efficiency so do not play a major roll in lean/rich settings.
These usually last the life of the vehicle because they get "cleaner" exhaust after Cat burns off the emissions.