How to calibrate the RTC accuracy?

[HFeng.2]

I'm using the STM32F767ZI MCU and with its RTC function. But my quartz crystal is not accurate. According to the document of the MCU, the accuracy can be calibrated (it said in page 35/255: Digital calibration circuit with 0.95 ppm resolution, to compensate for quartz crystal inaccuracy). But how can I calibrate it?

Thanks!