I'll say first - make a copy of the doc to play with - in case you find out that you've made a mess.
If the whole doc should have the same page orientation, columns, headers/footers, etc - you may wish to remove all section breaks.
Section breaks exist so you can separate chunks of the doc for such purposes as columns and etc.
An interesting experiment would be to select everything (ctrl-A) and press Ctrl-spacebar. This will remove manual formatting such as font size and selection.
Then select all and try Ctrl-q. This reverts all paragraph properties to their paragraph styles. (indents, numbering, etc.)



This may turn out to be a version of the doc that you throw away and go back to your original? That's ok, this is a learning experiment. Make sure to make a fresh test copy so you can blow it up again without losing anything.
But then you can apply styles and see what they do. And when you have a style applied in multiple places, you can adjust the style properties and see everything magically adjust to your new settings. Beats the heck out of manually formatting everything!

Oh, yes - while you're playing, turn on the display of para marks, maybe all formatting marks?
That helps you understand what you have in there, and can help you understand how to fix things.
Have fun in your voyage of discovery!