Mac Book not updated for a long time?

My mother and I shared a laptop. A Mac Book Air 2015. The only problem now is that software updates have never been done because we're absolute tech rivets. Now I have started my studies and need the functions of the laptop. How can I bring the laptop to the latest software?

You can just download it from the Mac AppStore

https://apps.apple.com/de/app/macos-mojave/id1398502828?mt=12

then open the downloaded app and install.

After the update to Mojave you will find the updates in the system settings

But I always have to be able to update on at least that and have the update

So as if it was necessary that I have to download every single update between my and the current one

What version do you have?