Windows 10 Bash (Ubuntu) - How to scroll up?

Right click the window frame, select "properties" from the contextual menu. In Properties, first tab, in the edit option area, tick the "QuickEditMode". That will allow you to mouse scroll.

Screenshot of properties:

You must set Properties → Layout → Screen Buffer Size → Height to the number of lines you want.

The default was 50 lines. I set it to 1000 and that's enough for me.