How can I change my visa appointment to another country?

Presuming that both of the US Consulates in the countries you're trying to change from and to both use the ustraveldocs.com/CGI Federal website (as most but not all countries do), then you will need to open a support request with them to have your "profile" changed between countries.

Alternatively you might be able to create a new account with them using a different email address, however if you've already entered your passport details on the website for the original country then this will NOT work without manual intervention, so it's not generally a good idea.

Note that any fee payments you've made will NOT transfer between the two countries, so if you've already paid the fee for the appointment you will need to pay it again in the new country - which may be difficult depending on which country it's in (eg, some require it paid locally using cash, whilst others allow the use of credit cards)