After I updated the Window 8.1, a"Secure Boot isn't configured correctly" watermark appears on the desktop. How can I remove it?

You can enable Secure Boot function in BIOS. please follow the steps below.
If your notebook does not support this function (there is no Secure Boot option in BIOS)
Please refer the fix path from Microsoft support site, http://support.microsoft.com/kb/2902864

Please restart your notebook then press F2 during POST to enter BIOS Setting.

2.     In the Security menu, please check if [Secure Boot Control ] has been disabled in the BIOS, and if so, re-enable it.

3.     In the Boot menu, please if [Launch CSM ] has been enabled, and if so, please disable it.

4.     Please press F10 to save and exit. After that the Windows 8.1 watermark “Secure Boot isn't configured correctly” on the desktop will disappear.