To edit one of your custom products, go to Vendors > All Products > Custom Items. 

This will show you a list of all of the custom products that your company has created.

Select the item you'd like to edit:

Clicking on the product name will open the item detail page - click 'Edit' to change any of the data attached to this item.

From here you can adjust pricing, manufacturer, model number, part number, labor values, product image, etc. Just be sure to click 'Save' once you are done. 

Next time you add this item to a project, it will reflect the changes you made here.
Note that these changes will not affect items that have already been added to projects in the past. If you need to edit an item that is already in a project, you can edit the line item within the project itself.

Let us know if you have any questions!