Where to change currency abbreviation
 

I'd like to change the currency abbrevisation. For example for $ i's like to change it to USD. Which files will have to be changed?

What do you mean here. We use the correct currency symbol where symbols should be displayed. The symbol is saved in the currency table in the database where it has the currency code which is USD and the currency symbol is $ as it is displayed.

In my ads it shows currency symbol followed by price. What I would like to have is cirrency code followed by price (USD10,000 and not $10,000) which files do I have to change? or is it better to alter the currency table in the database (that there will be no problem for future upgrades)?

You would have to change the symbol field in the currency table to USD