How do you reset maintenance required light 1990 Montero?

You should solve the issue that caused the warning first or you may be in for trouble down the road. You can pull the codes at most auto parts stores or a mechanic can do it for you.

That being said, if you pull the battery and turn the key to ON to discharge the power that should reset the light. If it returns immediately then you still have a problem.

Also see:

How_do_you_reset_the_Maintenance_Required_lamp_in_a_1994_Mitsubishi_Montero