went to costco to replace 4 tires,
they told me that the driver's side 2 of the wheel stud are
stripped, it is a drum brake.

what do you recommend to do ?

how easy it is to replace the studs ?

correction.

driver's side rear wheel

No big deal. You spray it with penetrating oil (not WD40) and then press it out the back of the hub with a special tool (a sort of gear puller) and pull the new one through from the back with an open-ended lug nut threaded on it. If you need to, place some washers on the new lug to allow the lug nut a stand-off so that it pulls the new stud all the way through. Torquing the lug nut on pulls it all the way in if it wasn't all the way in initially.

Some people say that you can hammer the lug out the back after treating it with penetrating oil, then pull the new one through.

As Brain stated its not a difficult repair. I'm surprised your tire dealer didn't take care of it for you, they probably stripped them in the first place. If they can't fix something a simple has a stripped lug bolt don't go back there again.