What to do if your manuscript word count is too low:

1. Consider making it book one in a series.
2. Publish it as a novella.
3. Reread your manuscript. Are there places you can add a scene? A chapter? 
4. Could you write a prologue? An Epilogue?

What to do if your manuscript word count is too high:

1. Reread your manuscript. Look for ways to use more concise vocabulary in order to say what you intend, but with less words.
2. Eliminate repetition. Especially in cases where you are revealing your character’s inner thoughts.
3. Split your book in two, making the second half another book in your “new” series.
4. Remove some of your content (especially in non-fiction or self-help books) and use the “extra” content in a blog, short story, etc.