Editing a drop down menu in a form can be done through the Form Builder on the Web Portal.

Web Portal > Forms > View Form Templates > Select Form you would like to edit > Form Builder > Find Section with the drop down menu you want to edit > click on the dropdown in the form, on the right hand side in field settings you will see the Items box, your list of dropdown answers will be available. They can be deleted, re-ordered, sorted alphabetically, edited (double click on word to be changed). New items can be added through the items box > Add > Save the form > Deploy / Publish Teplate. 

To ensure the edited form is up to date on the App, allow the app to sync on the device fully before going into the form.