It's pretty self-explanatory... You select your World Of Tanks folder, then select the server you would like to play on, and you press "Change Server".
If you would like to go back to your homeland server, press "Revert To Local Server".
Note: this program doesn't transfer your account to another server (it is impossible to do so). It just allows you to play on a different server, without being required to download the full client.
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Updated to v4.1
Fixed a bug
Credits: dimabal100000