You could always fit it with floats...!
Seriously; you need to amend the 'contact point' section of your aircraft.cfg file. Here's a copy of the section in my Concorde file, amended from the original FS2000 Concorde file.
[contact_points]
point.0= 1.000, 5.667, 0.000, -13.350, 1241.176, 0.000, 3.336, 34.080, 1.000, 2.500, 0.980, 5.000, 5.000, 0.000, 0.000, 0.000
point.1= 1.000, -54.000, -13.000, -14.250, 1574.803, 1.000, 3.336, 0.000, 1.500, 2.500, 1.000, 7.900, 7.900, 2.000, 0.000, 0.000
point.2= 1.000, -54.000, 13.000, -14.250, 1574.803, 2.000, 3.336, 0.000, 1.500, 2.500, 1.000, 8.100, 8.100, 3.000, 0.000, 0.000
point.3= 2.000, -14.167, -47.333, 0.000, 1388.141, 0.000, 0.000, 0.000, 0.000, 0.000, 0.000, 0.000, 0.000, 5.000, 0.000, 0.000
point.4= 2.000, -14.167, 47.333, 0.000, 1388.141, 0.000, 0.000, 0.000, 0.000, 0.000, 0.000, 0.000, 0.000, 6.000, 0.000, 0.000
point.5= 2.000, -116.667, 0.000, -0.833, 1574.803, 0.000, 0.000, 0.000, 0.000, 0.000, 0.000, 0.000, 0.000, 9.000, 0.000, 0.000
point.6= 2.000, 47.667, 0.000, -6.667, 787.402, 0.000, 0.000, 0.000, 0.000, 0.000, 0.000, 0.000, 0.000, 4.000, 0.000, 0.000
point.7= 2.000, 47.667, 0.000, 5.917, 1574.803, 0.000, 0.000, 0.000, 0.000, 0.000, 0.000, 0.000, 0.000, 7.000, 0.000, 0.000
point.8= 2.000, -125.000, 23.333, 0.000, 1574.803, 0.000, 0.000, 0.000, 0.000, 0.000, 0.000, 0.000, 0.000, 8.000, 0.000, 0.000
static_pitch= 0.220
static_cg_height= 13.243
If your Concorde is based on the FS2000 flight model, then cutting and pasting this into the equivalent point in the aircraft.cfg file should work fine (back-up the aircraft.cfg file first!) Points 0-3 define the parameters for the undercarriage, and the FOURTH figure (e.g. minus 13.350 on point.0 above) is the vertical distance in feet from the aeroplanes's reference point where the wheels will touch the ground. Alter this figure, and the aeroplane goes up or down.
Good luck!