Can I reprogram the Oscillator Calibration on a PIC® device?
The 8 MHz internal oscillator is factory calibrated. These calibration values are stored in fuses located in the Calibration Word. The Calibration Word is not erased when using the specified bulk erase sequence used by MPLAB® X IDE and thus, DOES NOT require reprogramming when the device is erased from MPLAB X IDE.
Otherwise, you can read or re-program the calibration word address with MPLAB IPE and a Microchip programmer debugger such as the MPLAB ICD4, or PICkit™ 3. Consult the datasheet of the part you are using to determine which address contains the calibration value.