Can I revise and export the same book again?

Yes. Once a book is unlocked, you can go back, change anything, and export again at no extra cost — that's the point of per-book pricing. Update your cover art, fix a typo in the interior and reformat, regenerate mockups — export as often as you need.

One thing to watch after a revision: if you change your page count in the interior, your spine width changes, so re-export the cover too. The tools flag this for you. See Why did my final page count change? and Can I change the page count after exporting?