If the difference between the set and real temperatures is within 35°F range then it can be adjusted.
If the difference is more than that, measure the oven temperature sensor resistance at the control board plug (terminals 1 & 2 at the five pins plug). The normal reading suppose to be about 1100 Ohms at the room temperature.
If the reading is incorrect, replace the oven temperature sensor.
If the reading is correct, replace the oven control board.
You can find the instructions in the attached mini-manual.
- The oven temperature sensor
Part number: WB21T10007 
- The oven control board
Part number: WB27T11313 
Here are the breakdown diagrams and
Parts for GE JBS55WK4WW Range - AppliancePartsPros.com
Gene.