First of all, even if it was just an example: Even if you can you shouldn't edit any pages with the name "posted" written in the page name. In some countries that may be considered as a financial fraud and can prove very problematic for your end customer on YE. There is a separate thread in the community discussing this
I would like to correct your example, unless there's a customization: After you approve a PO, in standard BC, the PO is set to "Released" status and not posted directly. If you wish to edit any details on the Posted Invoice, please do a credit memo which will nullify the invoice financial effects learn.microsoft.com/.../sales-how-correct-cancel-sales-invoice
Second, you will require a developer to make a field editable via Page Extensions
