text boxes and parent pages

Sorry for the delay -- time zones and the need to sleep occasionally.

I've looked at the file, and as @Robert at ID-Tasker notes it was created inan earlier version quite some time ago. The reason you can't select the text frame on the P2 Parent Page is because it isn't there. In fact, there is no frame for your body text on any of your Parent Pages.

Your text seems to be contained in a single thread that begins on page i which is based on the CH Parent, which in turn is based on the T parent, and neither of those has a frame for the body text, so it appears to me that way back at the beginning you simply placed the text and flowed it on the live page, so there is no way for you to make an adjustment in one place on the Parent Pages.

All is not lost, however. Go to File > Adjust Layout...

In the Layout Adjustment dialog be sure the Austo-adjust margins box is UNchecked and the margins are not linked (broken chan link icon), then increase the value for your top margin by the amount you were going to increase the frame inset.

Your running header and page number frames are on the Parent pages and can be moved there as needed. Your headers look like they will move with the top margin automatically, but that might not be where you wanrt them.

Also, I suspect you need to revisit your assigned page numbering. Page 1 is in the middle of the Ballad of the Scapegoat section, and your first chapter starts on page 3. Your other chapter starts may move as well after the layout adjustment, so you'll need to check carefully, and for overset text.