To save your Mix, simply go to File > Save or use the shortcut, Ctrl/CMD + S. Mixes are saved within projects, as seen in Project Creation.
To save your Mix in a different location than where it was first created, go to File > Save as or use the shortcut Ctrl/CMD + Shift + S.
You can also create a new Project to store your custom Mix in, from the + icon. Rename your Mix as required, and click on Save. The new Mix will be stored in the designated Project folder. You can also rename any existing Mix by Right Click > Rename.