You got 2 possibilities:
Combobox.SetCurrentValue[
^]
SetCurrentValue
sets the value of a dependency property without changing its value source.
or
Combobox.SetValue[
^]
SetValue
sets the local value of a dependency property.
Check other
Combobox[
^] properties (SelectedValue, SelectedIndex, SelectedItem) depending of your current .NET Framework version.