how much percent of marks deduct when I change stream from science to BA
As you have not mentioned the college name, I am answering your query with respect to the University of Delhi.
In DU, there is usually a deduction of 2.5% to 5% marks in its affiliated colleges when you apply for a course that belongs to a stream different from your stream at (10+2) level.
To get further information related to this query, kindly provide name of the college of your interest in the comment section.
Hope this answer helps you.