Keeping Linux Mint up-to-date is pretty simple –
Look for the little shield icon at the right hand side of the task bar (at the bottom of the screen)
If it has a little white “i” in a blue circle inside the shield then there are updates to be done.
Single left click on the shield and an “Update Manager” window will open.
Single left click on the words “Install Updates”
An “Authenticate” box will open.
Type your password and then single left click the “Authenticate” button.
Eventually the Update Manager window will say “Your system is up to date” in the bottom left hand corner.
Close the window (“x” in top right hand corner)
The little shield will now have a green tick in it.
(Occasionally you may not get “Your system is up to date” because there is a second round of updates to do. Just repeat the above until they are all done.)