The existing 10k input resistors need to be replaced by 1M resistors to better match the impedance of the guitar pickups. (I suppose you don’t need to change the second stage but I replaced both input resistors anyway).
On the underside of the pcb near the input phono sockets you will find two very small surface mount (SMT) resistors. These are clearly labelled 10k on the screenprinted pcb. Unsolder these and remove them completely then bend and trim the leads of two conventional 1Meg ohm resistors and solder them carefully to the small pads where the SMT resistors were.