how to fix my glowworm boiler f6 fault code?

Yes you can, but do you need to?

The Glow Worm help page says this for F6:

*The Heating Flow NTC Thermistor Has Gone Open Circuit (Should Read 12Kohms at 20C)
*Wiring Harness Fault Or Connection Fault Between The Heating Flow Thermistor And The Main PCB
*Flow & Return Piped Up The Wrong Way Round (Hxi Model Only)
*External System Pump Circulating The Wrong Way Around The System (Hxi Model Only)

So: check the connections, check the thermistor and only replace if it has failed.

Do, however, consider getting someone competent to look at it.

Ghiutdf
April 2018