The first think to check is the ST16 using Hardware Monitor. The sticks often get oxidation when it isn't used for a long time. Check it carefully and for at least 5 min. Look to see the graphs returning to zero when the sticks are centered and that they are smooth and go full scale. The drifting you describe is often caused by the sticks not returning to zero.
The GPS module is one of the easiest parts to change. You remove the 10 screws on the bottom around the outside of the body. Then carefully release the tabs at each motor arm to lift off the top. After it lifts, unplug the GPS cable and peel back the sticky shield which covers the GPS/compass module. I don't know of a Youtube video on this but I just posted a video last night on checking the GPS battery. You can see the location of the module.
The compass warnings became more frequent after the last firmware update. Generally after you get higher than 30-50' they go away and start again when descending closer to the ground. They also seem to occur when operating the landing gear. So.......make sure the problem isn't with the ST16 regarding drift and that the compass warnings are not related to ground effect before changing the module. BTW, I did the battery test video because of a related issue of the the GPS loosing co-ords and compass cal while in storage. The backup battery can run down when it isn't used for a long time.