To update dropdown and checkbox fields and add / save new options in your Web Portal > Forms > View Form Templates > Select the Form > Form Builder > Select the drop down you would like to have other options added to and/or saved > Select Allow other and Save other > To save the changes ensure you Publish & Deploy the form. 

'Allow Other' will allow the user to add an option that is not included in the list when filling out the form. 

'Save Other' allows any new item/s added via Allow Other to be saved to the list and can be used in future forms.

When allow other has been enabled it add in an additional text field below the dropdown/checkbox.

If you would like to add items into a dropdown permanently, you are also able to add them in the Form Builder. 

How do I edit a drop down menu in a Form?