Modify the material quantity or price in a posted purchase order

[DSK28DG]

In the SAP MM module, after a purchase order has been posted, how can I modify the material quantity or price in the already posted purchase order?

I am currently learning the MM (Material Management) module in SAP ERP, specifically the procurement process. I have already understood the basic process of creating purchase orders. Today, while posting a purchase order, I was wondering how to modify an already posted purchase order if there is an error in its content. I am using SAP Logon 750. I have read the relevant training materials and official documentation, but I am still unclear on some details. I would appreciate some specific guidance or best practice recommendations. Any related screenshots or step-by-step instructions would be greatly appreciated. Thank you for your help。