Close and re-open DH and it should return to default rate.
For now, you cant change the currency symbol($) . for some site that use a a different currency like Indian sites, it will track the hand based on the local currency but will show usd($) symbol.
Try rebooting your PC and if still does not return to default currency rating, please send us a message though support with screenshot attach so I can have our dev team check this for you.
You cant manually change the conversion rate. I was unable to re-create this issue. Can you please try reinstalling DH2 and see if the currency exchange reset.