You can enter the editing mode for a control template using the menu or using the resources panel.
Using the menu
Complete the following steps:
1. In the Objects and Timeline panel, select the control.
2. Do one of the following:
· On the Object menu, point to Edit Template.
· Right-click the control, point to Edit Template.
· Click Edit Current (if you want to modify the template that is currently applied to the control) or click Edit a Copy (to simultaneously create a new copy of the template that is currently applied to the control and apply the new template to the control). The Edit Current option is not available if the system template is applied to the control.
For an example, see Create or modify a template on MSDN.
Using the Resources panel (to modify an existing template)
Complete the following steps:
1. In the Resources panel, locate the style that contains the template, and then click the Edit resource button next to the style.
2. In the Objects and Timeline panel, right-click the Style elements, point to Edit Template, and then click Edit Current.
Control templates are wrapped inside styles so that the style that is applied to a control includes both the appearance (parts) and the behavior for the control. You click Edit Current instead of Edit a Copy because the copy of the style includes the template. To return to the editing scope of your main document, you must click Scope Up 
two times because the first click brings you back to the editing mode of the style.
Note: Styles and templates are resources that you can apply to controls in different ways and store in different locations in your application. For an example of creating a template, see Create or modify a template on MSDN.