In most cases the belt has streched out and need to be tightened. Be sure to check the belt condition for cracks and splits. On most vehicles the alterator is where belt adjustments are made. Loosen alternator mounting bolts, pry alternator in slot and tighten bolts while holding tention. AC belt may be a second belt and is tightened at ac compressor in the same method.
In some cases low power steering fluid will cause belts to squeal when turning.