If you need to chnage all the product prices by a percentage at once, here is an easy way to do that.
From your control panel, lauch phpmyadmin and select the database that your store currently uses.
Go to the sql tab and run the query that applies to you.
If you want to raise all your prices by a percentage (Example: 22.5%), run the query:
If you want to drop all your prices by a percentage (Example: 15%), run the query:
If the tables of you opencart database use a preffix (ex: in the table list, the table name would looklike: preffix_product instead of just product), dont forget to apply the preffice to the product table in the query too.