You cannot change the in-app store currency from your end after the fact. In the Steam version, Music Maker tries to pick up the currency from the currency configured in Steam. If that somehow doesn't work, or the Steam currency is not one that is supported in the MAGIX in-app store, you'll be given the option to pick the currency on your first purchase. If there's any reason to change the currency after that, you'll need to do so by contacting MAGIX directly:
is there regional pricing in Magix music maker in-app store ?
Depends on what exactly you mean by that. I suppose the prices are usually designed to come out at proper, round values for each currency, and will stay that way for a longer period of time. Now, that means that the price differences will not always correctly reflect the currency conversion rates on any given day. Sometimes, you may have a disadvantage, and sometimes you may have an advantage when comparing your chosen currency to another one in relation to the actual conversion rates between those currencies. So, I suppose, in this sense you could say that there's regional pricing.