First of all, find the leak. If the mattress is plastic, make a soap and water solution and apply it to the mattress with an eye for a tiny bubble where the air is escaping. Second step is putting a heavy duty packaging tape over the puncture or tear. You need thick tape as in clear security packaging tape. Make sure the surface is clean and dry before applying. If the mattress is cloth covered, finding the leak will be challenging but to seal the leak, you might try an iron-on patch with an iron to apply the patch to stick permanently.