How To Permanently Remove Windows Watermark : Activate Windows 

How To Remove Windows Watermark
Access Registry and change some configuration to remove the watermark
Right click on start and run or you can press windows key + r then type regedit
-Hkey local machine - Expand System - Expand Current Control Set - Expand Services - Svsvc
Right click on start or DoubleClick - change the value to 4
-Restart your computer