Login into your Dashboard and open the 'settings' tab at the top right hand corner of the page
Once there, scroll down to the API settings section and you will find a drop down menu, next to the encoding box, where you can select the default character-set encoding to be used:
Do not forget to 'save changes' once you have selected the character-set to be used as default for your account
Please note this is only applicable for SMPP.
Nexmo REST API uses WEB standard UTF-8.