Change of second language in 2nd PUC

Yes. You can change the second language from Kannada to Hindi in 2nd year. For changing the subject you have to approach the administration of your college and apply in writing for change of 2nd language. They will do it after taking consent from the university. But all depends on the college you are studying. If they don't have Hindi as a 2nd language it may not be possible.
But personally, I feel it is not good to change the second language in a 2nd year. You have to concentrate a lot to follow the subject. Additional efforts are required to cope up with the subject.

Electing subjects in PUC is a matter under the jurisdiction of state education departments. Generally the students will select it at the time of admission to class XI and then continue it till class XII.

In Karnataka state the option for the students is, for the first language - English and for the second language - it is one out of Kannada / Sanskrit / Hindi / Urdu / Tamil / Telugu / Malayalam / Marathi / French / German / Arabic.

If you are not feeling comfortable for a particular language then there is option to switch over and you have to find out from the administration of your college regarding the feasibility of doing it after completing 1 year. Accordingly you will have to apply for it as per the laid down procedure.

If you are not conversant with Hindi then you may face similar problem again. So you have to take a conscious decision based on these things as one precious year has already gone.