Details
Just like any other software installation, some modifications need to be reflected in the system, including but not limited to: New registry entries, Kernel modifications, DLL (Dynamic Link Library) files, and any software download.
If an update being installed is dependent on some of the running background applications or tasks, these tasks need to be terminated for the changes to take effect. Thus, a restart is required after the installation process where the system is free to change the relevant files.
Recommendation
Trend Micro highly recommends users and/or partners to save their work first or bookmark those opened tabs before clicking Restart Now. After restart, the security agent should be updated to its latest version.
If the pop up continuously prompts after a series of restarts, please contact Trend Micro Technical Support for further assistance.