To clean the backrest, first, please vacuum any dirt off the chair, then clean it with warm soap and water. After that, wait until it’s completely dry and you’ll be good to go.
In terms of the seat, you’ll also need to vacuum it first. Then, use Lysol or a deodorising spray on the fabric of the seat, and give it a good scrubbing with a moist sponge. Let the air dry the chair and you’re all done.