Take into account that starting on another server means that you're going to start the game from scratch.

When starting the game, you'll have two buttons, Enter and Server selection (the blue button with two arrows).

Server selection button will appear with the latest server you've played in, for example Zone S1. To change it, you need to click on the Arrows button. A window will appear and you have various options, such as Recommended and My Servers.

You should click on My servers and here you have all the servers you have progress or started to play.