Probably the official way to do it would be to uninstall and reinstall the game from the Gameforge Client.
If you're comfortable editing the registry, you can do it manually.
1. Exit the Gameforge Client (from the menu under your username, choose Quit)
2. Move the entire TERA folder (and all subfolders, as-is) to the new location
3. Run "regedit" (Start -> type "regedit" -> press Enter -> approve UAC prompt)
4. Find the following Registry Key: HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\Microsoft\Windows\CurrentVersion\Uninstall\{68f799ce-b2cf-44f5-8638-ce992d7fd0f4}
5. Edit the string value "InstallLocation" to your new location. (You can also edit the DisplayIcon string value as well.)
6. Start the Gameforge Client. In the TERA Settings, under General, you should be able to confirm that it's now pointing to the new folder.
