How change unit price of all item at once?

[HuguG7]

How can i change the values of all items from my store at once?

For example, i want to decrease the actual values off all prices, with less 5%.. how can do it without doing that, item by item on the cpanel of my prestashop? any free module? (other problem is my prestashop version..1.4.6.2 and cant accept modules that only works on 1.6.xx versions)