Firstly you can give it a light hand wash or you can give it good soak in the bath:
If your yoga mat is just a little dirty here and there get a clean spray bottle and add water with a little drop of very mild soap. Spray the solution you have made over your mat and leave it for 2 mins then take a clean dry cloth or sponge and wipe the whole mat buff dry with an old towel and then let in air for 30mind before putting it away.
Now if your yoga mat hasn’t been cleaned in some time and could do with a good clean you should start by filling your bath up with some warm water and some mild detergent. Once you have put your yoga mat in the bath take your sponge and start wiping away any dirt or marks, once it looks lovely and clean give it a good rinse with warm water then dry with an old towel and leave it out to air.