Before upgrading Kaspersky Endpoint Security for Windows to 12.6 version, please read the following information: https://support.kaspersky.com/help/KESWin/ReleaseNotes/12.6/en-US/Release_notes_kes.htm.
If you are using Kaspersky Update Utility to update databases and application modules, enable Kaspersky Endpoint Security for Windows 12.6 in the utility settings.