I wanna change my tires for snow tires. I don't want to buy them at my dealer but at Costco.
Can that void my Honda warranty ?
Your warranty is actually pretty hard to Void even though many people talk about it. The dealer will push back, but if you know your rights, you will win every time so long as you are within reason.
I'd suggest reading about the Magnuson–Moss Warranty Act which specifically states that modding a vehicle doesn't void the warranty unless that part can be proven to be the cause of a break down [sic]
So, no, changing out for snow tires will not void your warranty.
No, this should not void your warranty as long as the replacement tires are the same size as the stock ones. I'm not even sure the size really matters that much (within reason). Something most people do with winter snow tires is buy a set of separate wheels and have the snow tires mounted on them, that way when you go to change back, you can change them at home versus having to go back to the shop to have the regular tires mounted and balanced again. Saves a lot of time and money in the long run. These wheels could be cheaper steel wheels with hub caps to save money.