With the belt removed on a cold morning, spin all the pulley wheels by hand......replace any wheel that does not spin freely.
In the picture above, the pulley with the metal bracket is the belt tensioner pulley and the individual wheel by itself is an idler pulley.
Each wheel can be replaced individually. When buying the replacement wheel, it will come with new bearings. It is the pulley bearings that go bad which cause the squealing sound.