Click the "My Content" button in the content-spanning toolbar; select "My Page Sets...".
Observe the summary report listing all page sets that you own that are in the default subsite itself (not in any child sub-subsites); usually there are none.
Click on the linked number of Members, or select "Page Set Members..." from the pop-up menu of the adjacent tool icon, to add, remove, or change the ordering of the pages in the set, as described in steps 9 through 15, of the discussion of building page sets.
The tool icons at the right end of the row for your page set enable other tasks.
The rightmost item is a check-box; when one or more rows' boxes are checked, the "More Actions..." feature will let you delete, move, or change ownership of the selected page sets, all at once, with a single confirmation step.
Click on the "Close" button when finished with the "My Page Sets" lightbox.
If you need to work with a page set that isn't yours, click on the Tools button in the full-width toolbar and choose "Find...":
Click the "Show More" link in the lower-left corner.
In the "Show" section, click to un-check everything except Page Sets.
Select the appropriate subsite, and check, or clear, "Include Child Subsites," as appropriate.
Click on "Search" at the lower right.
Identify the page set you want to work with, and then use the appropriate tool icon at the right end of that row.