Unable to upgrade from MacOS Sierra to High Sierra

I decided to upgrade to High Sierra so went through 'Software Update' and got into the App Store where it said 'No App Updates Available'
It also says I have 4 Incompatible Updates in Keynote, Pages, Numbers and iMovie and looking at these they seem to require MacOS Mojave.
1) Has High Sierra been superseded by Mojave?
2) If so am I able to upgrade to Mojave and
3) Would that be a good thing to do?
4) If so, how do I access it as it doesn't seem to allow me to from the App Store or from About My Mac.