It depends on the style guide — "in" is lowercase in AP and Chicago but capitalized in NYT Style.
"In" is one of the most commonly debated words in title capitalization. Its treatment differs by style guide: it is lowercase in AP Style, Chicago Style, and MLA Style, but capitalized in NYT Style and Billboard Style.
Chicago Style uses a length threshold: prepositions of four letters or fewer are lowercase. Since "in" is two letters, it is lowercased in Chicago Style. MLA Style follows nearly identical rules.
When "in" opens or closes a title, it is capitalized in every style guide. The first-word and last-word rules override the preposition rules universally.
"In" can also function as part of a phrasal verb, such as "log in" or "give in." When a preposition is part of a verb phrase and carries verbal meaning, Chicago Style recommends capitalizing it. Context matters when applying the rules.
The capitalization of "in" is one of the clearest examples of how style guides differ. AP Style lowercases all prepositions, so "in" is always lowercase in journalism. NYT Style takes the opposite approach and capitalizes most prepositions, including "in."