changing dropdown menu field for every testing

Hi Guys,

what I want the system to do:

I will check a field, it is a dropdown menu, and if this field is for example A, I should select another option for example B for this field from the dropdown list. Or if this field is B I should change it to A so how can I do it with scripting? Do you have an idea? I could have not found the correct codes for this:(

For example in one test the field contains A and you should change it to B, than for the next time the field is B and you should change it to A.