We don't know HOW you have done it on the first place.
If you have worked consequently with Paragraph, Character and Object Styles, it is easy and a work for several minutes. You could even create an alternative layout, which would inherit any changes from the mane layout in text and other content.
But if you have designed your text manually, you will have a bunch of work in front of you.
If you just want a mini-version of what you have, place the current pages into a new file of the correct size and scale them.