Not sure if you really want to do that as you can always adjust both fuel and cargo by clicking on the change fuel payload section after selecting a aircraft. but if you do then this is where you would make the change
[WEIGHT_AND_BALANCE]
max_gross_weight = 20450.0                      // (pounds)
empty_weight = 11700.0                          // (pounds)

reference_datum_position     =   34.5,  0, 0    // (feet) distance from FlightSim Reference position, which is: (1/4 chord, centerline, waterline)
empty_weight_CG_position     =  -35.79, 0, 0    // (feet) longitudinal, lateral, vertical distance from specified datum

max_number_of_stations = 50

station_load.0  = 170, -17.0, -1.2, 0.0     // Weight(lbs), longitudinal, lateral, vertical positions from datum (feet)
station_load.1  = 170, -17.0,  1.2, 0.0     // Weight(lbs), longitudinal, lateral, vertical positions from datum (feet)
station_load.2  = 170, -22.5, -1.2, 0.0     // Weight(lbs), longitudinal, lateral, vertical positions from datum (feet)
station_load.3  = 170, -22.5,  1.2, 0.0     // Weight(lbs), longitudinal, lateral, vertical positions from datum (feet)
station_load.4  = 170, -26.7, -1.2, 0.0     // Weight(lbs), longitudinal, lateral, vertical positions from datum (feet)
station_load.5  = 170, -26.7,  1.2, 0.0     // Weight(lbs), longitudinal, lateral, vertical positions from datum (feet)
station_load.6  = 0,   -30.9, -1.2, 0.0     // Weight(lbs), longitudinal, lateral, vertical positions from datum (feet)
station_load.7  = 0,   -30.9,  1.2, 0.0     // Weight(lbs), longitudinal, lateral, vertical positions from datum (feet)
station_load.8  = 0,   -35.1, -1.2, 0.0     // Weight(lbs), longitudinal, lateral, vertical positions from datum (feet)
station_load.9  = 0,   -35.1,  1.2, 0.0     // Weight(lbs), longitudinal, lateral, vertical positions from datum (feet)
station_load.10 = 100, -42.0,  0.0, 0.0     // Weight(lbs), longitudinal, lateral, vertical positions from datum (feet)
station_load.11 =  50, -45.0,  0.0, 0.0     // Weight(lbs), longitudinal, lateral, vertical positions from datum (feet)
This is the Aircraft cfg file of the default Lear jet.
I must caution you changing anything here can cause major issues with aircraft performance I strongly advise you that you save the aircraft cfg file someplace safe just in case. because a little change here can have a huge effect.
Trust me it is better you leave this alone unless you know what you are doing.