Due to regulations on .BR domains, only the domain owner can apply changes to the service provider through Registro.BR.
To change your domain provider to Hostinger, just log in to your Registro.br account, and select HSTDOMAINS (127). You can see the detailed instructions in the video below:
Only the account holder can make changes
If you've recently changed your service provider, you'll be able to make the next change when it's released again - which can take up to 72 hours