During the DPM installation, the DPM setup creates two restricted local user accounts: Microsoft$DPM$Acct and Microsoft$DPMWriter$. The SQL Server and SQL Agent services use the Microsoft$DPM$Acct account, and the DPM Writer service uses the Microsoft$DPMWriter$ account. Also, DPM sets the SA password for SQL Server to the same password that you specify for the Microsoft$DPM$Acct and Microsoft$DPMWriter$ accounts.
Changing DPM SQL Service Password
It is like changing any SQL service password ( http://technet.microsoft.com/en-us/library/cc263226.aspx ). Change the SQL service account password using User Admin tool and update the SQL Server (MS$DPM2007$) and SQL Server Agent (MS$DPM2007$) with your new password using Service Snap-in. Start the service.