Question

how can you raise the pitch of the note of acertain veena string without changing the length

Answer 1

The pitch of the note can be raised by pressing the strings and than playing a note. In this way we do not have to change the length and a pitch will raise 

Answer 2

Veena is a string instrument. String instruments make sound with vibrating strings, and the pitch of a note can be raised by increasing tension in the string.