Note: This guide downgrades any version of WoW to version 1.0
First goto your World of Warcraft folder and open up your data folder
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Next select your patch files (Any that say patch in it) and delete them
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Now that those files are deleted you want to run the repair tool, just select the first option and go watch some TV becuase its going to take a bit.
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After that run whatever patch you want to patch to