Everywhere that you have the following code in your EditItemTemplate
s:
Text='<%# Eval("fieldName")%>'
You need to change it to this:
Text='<%# Bind("fieldName")%>'
Eval
is for one-way (read only) databinding. You want two-way databinding, in order to update the database values.
You also need to set the DataKeyNames
property in your GridView
:
<asp:GridView ID="grdBenutzer" runat="server" Visible="False" AutoGenerateColumns="False" DataSourceID="SqlDataSource1" OnRowCommand="GridView1_RowCommand" DataKeyNames="BenutzerID" >
This is required for the auto-update functionality to work, per the documentation (linked above):
Use the DataKeyNames property to specify the field or fields that represent the primary key of the data source. You must set the DataKeyNames property in order for the automatic update and delete features of the GridView control to work.