A sagging mattress cannot regain lost structure through a household trick, but you can temporarily improve support by checking the foundation, rotating when allowed, and using an appropriate comfort layer while deciding on repair or replacement.
Check the foundation first
Wide slat gaps, missing center support, or a deformed foundation can create or worsen sagging. Correct the support according to manufacturer specifications.
Rotate if the manufacturer allows it
Rotating head-to-foot can redistribute wear temporarily. Do not flip it unless it is designed to be slept on both sides.
A topper changes comfort, not structure
A topper can soften the feel of minor unevenness, but it will follow a deep sag. Do not use it to hide a major support failure.
Measure the sag and check warranty terms
Many manufacturers define a specific sag depth for warranty claims and require an approved foundation. Document the problem before making permanent modifications.