Once a Direct Fulfilment, Self Fulfilment or Digital product has been ordered or is in the process of being ordered by your customers, it will no longer be editable by you.
This is done deliberately to ensure that your customers retain a consistent experience in their shopping cart and so that your historic order records are kept intact and avoid data corruption issues.
A locked product can be deleted from your account, however this will simply hide it from your view. Historic and in-progress orders will still retain the deleted product information.
If you wish to offer an updated version of the product, it is best to delete the original (locked) product and create a replacement product that you then add back into the relevant catalogs.