First things first. Word counts aren’t written in stone. Many editors and agents would rather read a solid story that weighs in at 57,000 words than a weak story of 73,000 words.
Now, if you need to round out a word count, there is an ‘easy’ fix and it’s something you should do anyway, too. Read through your story and look for where you’re telling instead of showing. That’s a common problem and it could help tilt the scales in your favor.
However, that alone won’t get you the numbers you need. Unfortunately, adding 20-30k words without filler usually involves big changes to your plot. Proceed with caution if you go this route.
My best advice to you would be to focus on telling the best story you can, word counts be damned. Some stories work well as novellas, and fail terribly as novels. Don’t force your story to be something it’s not.
-Graphei