Alexey,

Go to the Windows Start menu > All Programs > PBP3 from melabs > PBP Activation Manager.
Check the Deactivate checkbox, and click the Deactivate button.

Open the activation manager again and re-enter your name, email and activation key.
Press the Activate button.

If you don't have your activation key, you can log into your account at melabs.com and go to Order history.