The Husqvarna HU700F lawn mower uses a Honda GCV160 engine, which means the air filter is on the right side of the engine as you are looking at it from the front of the mower. It is encased in a black housing, and is easy to remove for cleaning or replacement. Husqvarna recommends cleaning it every 25 hours of operation and replacing it every 100 hours.
To take the air filter out, simply pop out the two tabs located on the top of the black air filter cover. The cover will swing open, revealing the flat air filter. Remove the air filter, and either clean it or replace it with a new one. When installing a new air filter (or reinstalling the now-cleaned one) place it in its housing with the pleated side facing out (away from the engine). Then push the air filter cover closed so the two tabs lock into place.