You can start by selecting <File> <Titles and Sheet setup> and select a landscape orientation sheet of a size you think it will all fit on.
At this point, if you can cut wires that go to the "upper" section, that would be a big help.
Select the "upper" section to be relocated to the right of the existing schematic "lower" section, preferrably with no wires going to that section.
This part is tricky ... to move the section you just selected, you must move your mouse over the chips until one of the chips highlights. If you don't do this you will be dragging parts instead of a group. I think in the default, a chip turns a bright, cyan color.
Fix any connections that you had to cut.
It's always best to save a backup copy before major surgery
Note: I've tried to use the "group" command to do the selection process but was unable to select the group afterward for some reason.