I booked Turkish Airline from Accra to Istanbul and Istanbul to Accra.
But I am Argentina passport holder and visa free to Europe, can I change my return ticket from Istanbul to Paris or Berlin instead of Accra??
We don't know what fare you purchased or what terms it carries. Change fees and policies vary depending on the fare you paid. The only way to find out is to contact your airline or travel agent and ask them to price the change, which will also depend on the exact dates and times you wish to travel. You will likely have to pay a change fee and any difference in fare.
It's also worth pricing out whether it is cheaper to purchase a new separate ticket to Paris or Berlin instead of changing your return ticket, e.g. on a low-cost carrier like Pegasus out of SAW airport.