For Plesk version 11 & 10
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1. Log into Plesk control panel.
2. Expand "Server Management" section and select "Tools & Settings" option.
3. Select "Interface Management" option from "Panel Appearance" section.
4. Select the desired interface and click "OK" button.
For Plesk version 9
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1. Log into Plesk control panel.
2. Select "Settings" from "Main Menu".
3. Select "Interface Management" option from "Control Panel Interface" section.
4. Select the desired interface options and click "OK" button.