There's no difference between BullGuard trial and BullGuard "paid". In fact, both applications are the same. The only difference is the length of the account subscription: trial or paid.
Therefore, you can activate your license key on the already installed application:
1. Double-click the BullGuard icon near the clock (lower-right corner of your screen).
2. Go to the Main section > Account tab.
3. Press the Enter License key option under your username.
4. Activate your license key