Depends on how far you want to go. Bad schematic so I take no responsibility if these are the wrong part numbers but you can change R5, R7 which are 220k plate resistors on the first two gain stages to 100k and R44 which is the cathode resistor on the first stage to 1.5k from 3.3k. The input resistors are 100k, might make the tone a little darker or not, 68k is normal for most amps.
If you want variable tone rather than a switch you can change R5 and the switch to a 500k pot. That will get you most of the way there.