Editing Drop Down Menu in Personal Monthly Planner template

I have downloaded the monthly planner but found that it did not contain all 12 months in the drop down menu at the top of the form.  I have read the Help topics on creating and editing drop down menus but, as I didn't create this template, I don't know where to find the worksheet that created the original drop down content.

How do I add more months to the drop down?  And, for next year, how do I add the 2023 months and align the start date with appropriate day of the week?

Thanks all!