See there are no notification available online for the 1st PU subject change procedure in the official website. I would recommend you to contact your college authority for the same and they can only guide you out as they have the information from the board.

TO know more about 1st PUC, visit the link below:

https://school.careers360.com/boards/dpue-karnataka/karnataka-1st-puc-result

To know more about the career in Commerce, visit the link below:

https://www.careers360.com/courses/commerce-course

I hope this answer helps. All the very best for your future endeavors!