Hello aspirant,
It is not possible to change your stream in class 12th.
You may change a subject but nit a stream.
11th and 12th are connected, the syllabus might change but once chosen a stream in class 11th you cannot change in 12th.
If it becomes very difficult then you may repeat class 11th with arts.
All the best.

Hello dear student, 
Greetings
Class 11 th and 12th are connected and is together known as intermediate.  You can select your stream and subjects only in class 11 and then you cannot change it in your class 12th . So , dear now that you read Science in your class 11 , you cannot switch to Arts  stream now unluckily.
thankyou
Good luck