C# VSTO Excel Addins: How can I access Ribbons Controls from the Addins ThisAddIn_Startup?

I'm designing an Excel Addins with Ribbons like this:

• The Ribbon Name is Ribbon1
• The Tab Name is Tab1
• The ToggleButton name is tglShowNavigation

What I need is:

From the ThisAddIn.cs, in the ThisAddIn_Startup void, I would like to be able to Affect/Change the ToggleButton tglShowNavigation's Label.

I tried the following but cannot see any control from this:

Where and how could I access the Controls on my Ribbon from the ThisAddIn_Startup please?

NB: I don't want to call a global function from the Ribbon, but just want to be able to access them directly.

Thank you in advanced for you precious help!