How to cancel pending update?

I would rather stay with my initial installation.

Apparently, according to a thread on our old forum, if you set updates to Manual, nothing appears until you search for it or set updates to Automatic. I can’t actually verify this, but if you go to Settings/Sailfish OS updates; under Settings submenu, it shows ‘Check for updates’, set it to manual and see what happens. This is not cancelling it, just makes it not visible to you in the UI.

Exactly as @Edz said.
Just select manually and this nagging notification hint will be gone.
You will still see in settings that there is a new update available and you can always search for newer (stop) releases manually.

Actually you can’t get rid of the notification in the event view if that is the only notification you have. However if you also have a second notification, you can long press on that one and both will be selected, so you’ll be able to swipe both away. Tested on 4.2, Manual (the notification appeared after I manually checked).