Hi @Ahmad_R_Beigi,
Changing the default character set is more of a MySQL question than an OJS question; see e.g.: How to convert a MySQL database to UTF-8
However, as you work through this, be very careful to make sure that you aren’t introducing a mix of data in different character sets – this can be very difficult to disentangle. If your data is in one character set before upgrade, and another character set after upgrade, then converting between those two configurations will always leave part of your data incorrectly encoded.
The best solution, if it’s possible, is to re-run your upgrade with the right character set configuration. This will ensure consistency. This will depend on whether you have new data created since the upgrade that you need to preserve, however.
Regards,
Alec Smecher
Public Knowledge Project Team