And you need to think ahead to the next step, what if it is indeed off? Can it be calibrated, do you have the service manual or the tool to do it?
Most cheap meter does not have a way to calibrate resistance measurement, even it is off, there is not much you can do unless you would do mental compensating calculation on every reading. These are throw-away meters that are built to a low price point.
I suggest you first change the battery to see if it makes any difference, check for failing probes and wires as others have suggested. If not, buy a second good meter and compare the reading. If it is off, then I would suggest to retire it or pitch it.