Currently. No direct account change option is there in app. But you can follow below steps to change backup account.

– First Take backup of application in mobile and note down the file name.
– Now,uninstall and Re- install application from playstore.
– Restore your data from mobile as below:
>> Click on restore.
>> Select mobile(as previously you have taken backup in mobile device)
>> find out sales assist folder and click on SalesDatabase.
>> Click on last backeped database to restore your data.
>> Now, when you take backup in drive it will ask for account, So that time you can set your new account id for backup.

Note: Before uninstalling appplication, first check your backup succesfully taken in mobile by checking file name and then uninstall it app.