Sagging gutters can be an easy fix depending on the severity / condition of the existing material.  Hidden hangers in the appropriate size (5" or 6") can be used to lift up the sagging areas in order to secure to the fascia boards and tighten as needed.  If the gutters are severely warped or bent from ice dams and / or debris, then replacement is recommended.  Contacting a professional to take care of this service is recommended unless the homeowner has experience working from ladders in order to avoid injury.

Eric, Arocon Roofing & Construction