A new belt will often stretch and 'settle in' a little after the first
few hundred miles.  It just needs tightening a little.

Jack up the car and SECURELY position SAFE supports.  You are going to
be working under the car.  How's this for a visual:  frequently people
don't die by being crushed; they die after the load is taken off them,
and they quickly bleed to death internally.

Pull off the wheel and take off the splash shield to gain access to
the belt.  Loosen the idler pulley (the bolt in the middle of it).
Tighten the adjuster from above.  Don't overdo it, it stresses all the
bearings of the accessories.  You should still have 1/4 inch or so
deflection in the longest span of the belt with moderate thumb
pressure.  If it continues to squeal, then the belt is now damaged &
must be replaced.

Retighten the idler bolt.

JM

My instructions are for the 4th gen Maxima 95-99.  But I went back &
re-read your posting & you didn't actually specify what year.

What year is it, someone here can help.

JM