* By turning the tuning pegs. Each string on a harp is attached to a tuning peg. By turning the tuning peg, you can tighten or loosen the string, which will raise or lower the pitch.
* By using the levers. Harps have levers that can be engaged to change the pitch of the strings. When a lever is engaged, it shortens the length of the string, which raises the pitch.
The levers on a harp are typically arranged in rows. The levers in the lowest row raise the pitch of the strings by one semitone, the levers in the second row raise the pitch by two semitones, and so on.
By using the tuning pegs and the levers, you can change the pitch of a harp to play in different keys.