Freezer not getting cold enough

My freezer creeped up to about 15F over several days. I decreased the thermostat setting from 0 to -6 and the temperature went down to 9F and is holding. Then it went down to 0 and a day or so later was up to 10F again. Refrigerator appears ok. I have not done anything else but have seen several threads for similar issues but different models. Can you help me troubleshoot this model?

Thank you.

That is 48 degrees in the freezer, convert to farenheit.