Update multiple rows combobox selected item

I have datagrid with multiple rows and I am working in WPF.Each row has a combobox.
I have selected a few rows and I want to update the combobox selected item data to all the other row's combobox after selecting the combox item from the dropdown.

The steps are :

1. Select the rows to update .

2. Click the Update button .

3.Now the combobox's selection  of  first selected  row has to be  changed to desired value.

4.The selected value has to updated to all other selected rows combobox selected item data.

I am able to get the selected the rows but not able to make combobox to select a value and get the selected value.