Dynamic change of content of text fields

Have a form with calculations and currency codes, but the currency codes do NOT actually pull in any live data, they are there merely to indicate what currency is being used.

We need a way to manually enter the currency code being used in one position on the form and then have it update everywhere else in the document OR using a dropdown, choose the currency code and then have that update/change the currency code wherever it appears (see attached).

Any ideas?

Never done any scripting so if that is involved will need some very simple step by step instructions please.

Thanks in advance!