Example
the lord of the rings: the return of the king
The Lord of the Rings: The Return of the King
Which words stay lowercase
Articles (a, an, the), short conjunctions (and, but, or, nor, for, so, yet), and short prepositions (at, by, in, of, on, to, up, via, with, from, and similar) stay lowercase unless they start or end the title or come right after a colon.
This follows the general approach of Chicago style. APA and AP capitalize prepositions of four or more letters, such as “With” and “From”, so check your style guide if you need exact compliance.
Hyphenated words
Each part of a hyphenated compound is treated as its own word, so “state-of-the-art design” becomes “State-of-the-Art Design”.
Changing case in Word, Google Docs, and Excel
Microsoft Word: select the text and press Shift+F3 to cycle through lowercase, UPPERCASE, and Capitalize Each Word, or use the Change Case (Aa) button on the Home tab, which also offers Sentence case. On a Mac, use Format › Change Case.
Google Docs: select the text, then choose Format › Text › Capitalization.
Excel and Google Sheets: use =LOWER(A1), =UPPER(A1), or =PROPER(A1), then paste the results back as values. For anywhere else, such as email, a CMS, or a form, paste your text into this converter.