How to change timing belt on a 90 Maxima?

Far too complicated for a simple answer. Buy a manual. I've done it twice using a combination of Chilton's and Haynes. They aren't the best manuals, but they will get you thru this job.

You'll need a torque wrench.

And don't just do the timing belt. Do all the belts, plus the water pump and thermostat. And check the cam seals and crank seal while you're in there.

Bill G '91 SE Auto

Get a manual, use a dealer t-belt. It has marks which make it less likely you'll install it out of time.