Using a solder sucker / copper braid / a brillo pad (whatever works for you) remove the existing mosfets for the DC-DC converter. Try not to destroy the pads on the board!
carefully reinstall you new fet's. re-solder back in and reattach to any heatsink.
at this point (when everything is disassembled) its a good idea to give it a trial run.
1. make sure the surface your working on is free of conductive debris.
2. attempt to power up the inverter. (use a current limited source like a power supply/ wall wart if possible, not a car battery)
3. carefully check the output voltage (set meter to vac and check the output side)
4. if the output is 100VAC, try a simple 100W lamp.
5. note, i take no responsibility if you blow up you laptop/ house/ solar-system by following this instructable. prolly not a good idea to plug in an expensive laptop, but a blender / lamp / electric pencil sharpener / your 386 can take a beating and are less finicky to odd power supplies.