If all of those Products use that Category's categories_id as their master_categories_id ...
You can backup your database and then run:
Code:
UPDATE products SET products_price = products_price * 1.15 WHERE master_categories_id = XX;
where XX is the master_categories_id for these Products ...
Then, go to the Tools ... Store Manager ... and click admin/store_manager.php, and run the:
Code:
Update ALL Products Price Sorter
to be able to sort by displayed prices: Update