May I know why you want to join Science?

Don't underestimate yourself by taking Arts, It has lots of benefits, You can get various job in Arts too. Still if you want to join Science, Then consult your college, They will convey the board and then only it can be changed.

Note that: Most of the boards allow stream change within 3 months of admission, So please specify your board so that, I can explain the process better.

But don't regret, Every stream has its own quality and Arts has too. There are many career opportunities available in Arts too.

Bye and Have a great future ahead my friend.

Keep studying...