I looked there before posting this question. What I found was this:
How to apply filters
1. To be added
How to keep your system up-to-date, including the Kernel
Synaptic provides two methods for upgrading your system:
Smart Upgrade (Dist-Upgrade) -- recommended
The smart upgrade method tries to resolve package conflicts. This includes installing additional dependencies (required packages) if needed and preferring packages with higher priority. Smart Upgrade has the same effect as the apt-get dist-upgrade tool on the command line.
The default upgrade method marks upgrades of installed packages only. If the new version of a package depends on not installed packages or coflicts with an already installed package, it will not be upgraded.
IconsPage/eyes.png Dist-upgrade is the default upgrade method used by Synaptic. To change the upgrade method, choose Preferences from the Settings menu, then click on the General tab and adjust the System upgrade entry.
To upgrade your system with Synaptic:
Click Reload or press Ctrl + R to make Synaptic aware of the latest updates.
Click Mark all Upgrades or press Ctrl + G.
Click Apply on the toolbar or press Ctrl + P. A dialog appears with a summary of the changes that will be made to your system.
Click Apply to confirm the changes and go ahead with the upgrade.
IconsPage/info.png Synaptic works hand-in-hand with Update-Notifier and Update-Manager to inform you about updates to the software installed on your system.
Update-Notifier displays an icon in the notification area when updates are available. It will display a pop-up message and an icon in the notification area on your GNOME desktop.
But nothing about using the '[!]' filter to upgrade packages. Evidently, Synaptic in this version of Linux Mint Mate does not have the
Mark all Upgrades
icon / button...at least I didn't see any (which is usually located up top).