RF Explorer Combo Signal Generator Calibration

I’m using a RF Explorer Signal Generator (RFE6GEN) with an Expansion 0.1-6000MHz board and I’d like to know if there is a way to calibrate the Frequency accuracy of the unit. According to the RF Explorer Signal Generator User Manual, if I read this well, the Specifications for the unit shows:
100KHz to 250MHz: ±10ppm
250MHz to 6GHz: ±1ppm

I use a GPS clock as a frequency reference and I’m getting the following accuracy results:

Frequency Accuracy
5.8 GHz 1.82 ppm
1.8 GHz 1.6 ppm
840 MHz 1.26 ppm
440 MHz 1.1 ppm
220 MHz 12.5 ppm
140 MHz 20.0 ppm
52 MHz 58 ppm
28 MHz 100 ppm
14 MHz 220 ppm
7 MHz 440 ppm
1.3 MHz 2000 ppm

Could you let me know how I can improve the frequency accuracy of my Combo Signal Generator to match the published specifications in the User Manual.

Thank you in advance for your support.