How can we alter the pitch of guitar

Tuning the keys.

Explanation:

"Guitar strings are tuned (tightened and loosened) using their tuning keys. Applying too much tension to a string tightly can raise it to the pitch of the next note, while loosening it can easily lower it the same amount. Increasing the tension raises the pitch. The length of a string is also important."

I hope this answers your question! Have a good night!

d. air pressure decreases as altitude increases.