How do you change WoW to patch 1.21

The only way to do this is to reinstall the game from disk (patch 1.1) and manually run each and every patch up to patch 1.21 and keep it that way.

Note that if you do this, you will not be able to access the Blizzard game servers until you have patched up to the current patch level. This will only allow you to access very outdated classic private servers.

It is not possible to downgrade a fully patched installation of WoW down to 1.21 again. You can only grade up, not down.