How to say 'relax' in Spanish: Relajar. You try it. Make sure the J-sound in the word is a 'hu' sound. You try it.
If you want to say something like 'I love to relax at the beach', you could say, "Me gusta relajarme en la playa." You try it. If I ask you '?Te gusta relajarte in la playa?' 'Do you like to relax on the beach?' you would say, "Si, me gusta relajarme en la playa." Try it. ?Te gusta relajarte en la playa?
That's how to say 'relax' in Spanish.