modify regular price before storing in database

I want to modify the regular price of my woo commerce in PHP but I can’t find this PHP file. I can’t find where it is. I m new to WordPress. basically, I m an asp.net developer and I have done the same thing in asp.net MVC but I can’t find the regular price here. I just want to add 2.5% to the regular price. and I want to modify this file.
like regular_price+=regular_pric*2.5% and then save it to the database.
Please guide me step by step.
thanks in advance.