Unzip the hot tub cover and pull out the foam. Put the foam aside temporarily and let air dry. Turn the hot tub cover inside out and hang over a clothesline or fence.
Put on rubber gloves and pour bleach into a bucket. Submerge a scrub brush in the bleach and scrub the inside of the hot tub cover as it hangs. This will kill any bacteria, algae and rid the hot tub cover of any dirt and grime. Let the cover air dry in the sun.
Purchase foam panels from a local hardware, home improvement or pool and spa supply store. These panels are generally four-by-eight-foot panels.
Measure the old foam panels once dry with a tape measure, then cut the new foam panels to the same size.
Wrap the new foam panels in polyurethane wrap and tape the panels with silicone tape. Both the polyurethane wrap and silicone tape are waterproof and will keep the foam dry.
Turn the hot tub cover back in once dry and stuff the newly wrapped, taped foam panels into the cover. Zip the cover up and use as normal.