In 'handling.meta', lower the 'z' value for '<vecCentreOfMassOffset' (can be negative if required)

<vecCentreOfMassOffset x="0.000000" y="0.000000" z="lower this value" />

Lower it in steps of ~0.5 until it stops tipping over & then edit in smaller increments (up or down) until you find a nice value for it.
Generally, somewhere just below the point where it stops tipping over is usually best. A bit too low & the vehicle will hug the ground unrealistically & lack any roll. Much too low & the the vehicle will tip over into the corner rather than to the outside/lift up at the back on acceleration/wheelie on braking etc.