Step-by-Step Guide to Manual Activation of Unity
1. Start up your Unity Engine.
2a. (Windows) Click the Help tab and then click Enter serial number...
2b. (Mac) Click the Unity tab and find the Enter serial number... drop-down.
3. Click on Reactivate. 
4. Select Manual Activation. 
5. Click the Save License Request... button. 
6. You will then have the option to save the .ilr file to your hard-drive. 
7. Once you have the file saved, visit https://store-unity.unity3d.com/manual/. Upload the .ilr file generated in step 6, along with your Serial number for Unity. Click Activate Unity. 
8. You should then receive a .ilf in return. Again, save this to your hard-drive.
9. Switching back to the Editor window, you should now click the Read License... button. You should then select the previously saved .ilf file. 
10. Click Next. You're now active! \\