How to update value in one field based on a dropdown selection?

If drop down box 1 = 23 can text box 2 = X 

Kind of like the conditions for emails, but on the form it self. So if a user selects X from a drop down box the next box is auto populated with an answer of my choice? 

Yes, that is possible.

You can do that in two ways:

1st - using Form Calculation

If your field that should be updated is only used to show something based on the selection in the Dropdown field, I would suggest using Form Calculation widget. It will capture the value in the dropdown and show it within itself.

Useful guides:

How to Assign Calculation Value

How to perform calculation in the form?

2nd - using conditions

Conditions can be used for lots of things and the one you would need here is the Update Form Field Value option. It will allow you to check the options selected in the dropdown and then insert a value into some field

Useful Guides:

Smart Forms Using Conditional Logic

How to Insert Text or Mathematical Calculation into a field using a Condition

Of course we are here, so just let us know if you have any questions along the way and we would be happy to assist.