Sometimes the snap-store process starts in the background and stays open with 0% CPU usage even if the user did not actually launch the Snap Store application in the current session. For example, this would be the case if the following notification appeared on your desktop one or two minutes after booting Ubuntu.
If you opened the System Monitor it would show that the snap-store process had started automatically and was running. Ending the snap-store process would enable you to update the snap-store by running the following command, or alternatively a notification would appear that the snap-store has been updated would appear immediately after the snap-store process was ended.
sudo snap refresh
Alternatively to end the snap-store process so that the snap-store snap can be updated run the following command.
snap-store --quit && sudo snap refresh snap-store
snap-store --quit doesn't work then force end the snap-store process from the System Monitor and run
sudo snap refresh snap-store.
After the snap-store has been refreshed you may receive a notification in the notification area next to the clock that says
"snap-store" snap has been refreshed. Now available to launch.