One of the ways to edit data is by using the Edit Dialog functionality. You can launch Edit Dialog by selecting a row and then clicking on the "Edit" button in the grid toolbar (bottom left in this example). For this, you need to have navigator enabled.
As of version 4.8 it is possible to customize the form in a way that you want. To place the jqGrid editable fields it is needed to enclose the name form colModel in {}
Also you can add a custom button and attach a code to it when the form is created.
Please, take a look at the code in the HTML tab for additional details.