How do you Change front main seal on 92 Nissan 2400?

You would have to remove the timing cover (and everything in front of it,....fan shroud, fan, fan clutch, water pump), all the pulleys and brackets including the A/C compressor (if equipped), but you should be able to just move it out of the way,....you shouldn't need to discharge the system. After that....you need to drain the oil and drop the oil pan to get to the bolts from under the timing cover. The seal is pressed into the timing cover from behind.