If you would like to update the price of several parts by a percentage then please go to Maintenance ->Inventory and then press 'Update Parts Prices where you will see the following (it is strongly recommended that you take a database backup):
Please note that only the selected rows will be updated. Once you have pressed 'Yes' you will see:
In this screen we have selected to update the retail price of all parts items by 4% and labour items by 5%.