If you've tried restarting and the App Store still won't let you update, try installing the update manually via the command line utility. To do this, open Terminal and enter these commands, pressing return after each line and entering your administrator credentials when prompted:
Code:
sudo softwareupdate -da
sudo softwareupdate -ia
Before entering the second line, wait for the line ending with a $ to appear before entering the next line. Note that these commands will install all available updates, if you need to avoid an update for another application for some reason, then run just the first line, then try installing it from the App Store again. (The first one downloads all available updates, and the second one installs all updates.)
Quit the App Store and try running the below command in Terminal. This will remove a couple settings related to the App Store and device-specific application settings, then restart and try again.
If that doesn't work, reboot your computer into macOS Recovery, open Disk Utility there, click View » Show All Devices, choose your internal drive on the left (not the individual partitions), then click First Aid » Run.