Learn Russian: Какая это буква?

The question word какой (which/what kind of) changes its ending to match the gender of the noun it asks about. Since буква is a feminine noun, we must use the feminine form какая.

If буква is feminine, why do we use the neuter word это instead of эта?

In this sentence, это acts as a fixed pointing word meaning "this is" or "is this". When used as this general pointer to identify something, it always stays in the neuter form это, no matter the gender of the noun.

Does буква mean a letter you send in the mail?

No, буква specifically means a letter of the alphabet (like A, B, or C). If you are talking about a letter you write and send to a friend, you need to use the word письмо.

Can I change the word order and say Какая буква это?

People would understand you, but it sounds unnatural. In Russian, the standard pattern for asking "Which [noun] is this?" puts это directly after the question word, making it Какая это буква?