Calibrate the altimeter. See the user manual for the model you own, if you do not know how to do it. We cannot tell you better, since you did not tell what your watch model is.
Have you been outdoors, and preferably on a flat terrain, in a safe distance (~50m) from buildings or terrain irregularities when calibrating? Also note, that the reported elevation will change with any air pressure or temperature change. So if you changed the environment after the calibration, entered or exited a building, turned on/off the AC, or started an intensive workout, hence heating up the watch, the elevation would change too. You need to recalibrate the watch when the conditions change, and when calibrating before an activity you need to warm up before doing so.
Also, use the DEM (Digital Elevation Model) method, not the plain GPS. The vertical accuracy of GPS is almost an order of magnitude lower than the horizontal resolution, it means a deviation of many tens of meters is well possible. You have to have a phone with active data connection connected to your watch near you, when doing the DEM calibration.
