What do I do with R1 and R3 in order to get +6 dB gain?

R1...R4 seem to set the gain. Now my guess would be that you relocate the two resistors that occupying two of the four positions. So moving these resistor to the corresponding positions would give you the gain.

Move the resistor in pos R4 to pos R3. (0dB -> +6dB)

No promises... 😉

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thank you for your reply.
and you're right; the seller replied with the same solution.
But the pcb is about 3/4" by 1". I don't think i could get in there if i were a 6" man!

there are little holes in that pcb which might be the answer. but they're not located in positions that'd make cross-wiring feasible.
 
If I understand what you meant, you couldn't 'cross wire' anyway, because R4 still needs to be removed.

The little holes are vias connecting one side of the board to the other.

If you can't manage moving the SMD resistor, you may be able to remove it and use a through hole resistor suitably bent to fit (and possibly insulated), although it'd be awkward!