I have got visa, but i have changed my mind to do graduation first in my home country. After graduation, if i apply for study permit will i get approval or will face difficulty in future?
I think that is perfectly fine as long as you will still be eligible for the visa when you re-apply. I would inform the IRCC via a web forum before embarking on that though.
It will all depend on your circumstance in the future when you decide to re-apply. But not using this visa this time should not negatively affect future applications.
As advised above, inform IRCC you intend to cancel your current visa.