You can take them to any tire shop that offers it, the best way by far is when they mount the tire to use pressurized nitrogen though.
Instead of just depressurizing them and re-pressuring them with N, most of the air will still have humidity and O in them. Best to find a place that uses N in the machine that sets the bead also, it purges a lot of the normal air out when they do it.
True story, I needed to add “air” to my tires so I plumbed my TIG welding line to a tire filler and popped it on the tire, then just dialed the pressure off the argon tank and filled them with my welder.
My old man walked by while I had the welder on and stepping on the pedal and thought I was nuts, but it worked just fine.