A howto for changing the Network Access Account in SCCM 2012.
Go to Administration -> Site Configuration -> Sites and select the server on the right side.
Go to the ribbon Settings -> Configuration Site Components and choose for Software Distribution.
Click on the tab Network Access Account
Change here your Network Access Account.
(I’m using Administrator for Network Access, but this is not the correct way to use administrator for NAA. You have to make a new account with share rights (local admin on SCCM server of special rights for the shares) )
Click Ok and you are done.