You could do this with an UPDATE command via the Insert SQL Patch if you know the master_categories_id ...
Code:
UPDATE products SET products_price = products_price + 5.00 WHERE master_categories_id = 32;
NOTE: be sure to backup your database before attempting this ...
When done, you would want to run the Tools ... Store Manager ... for updating the products_price_sorter ...
Also be aware of the implication on Specials ...
admin/store_manager.php