default schema (or default sql id) is always the userid. Thats it.
To change it after connecting you have to set it.
Whats wrong that you can't do it ?
What application interface are you using?
In ODBC there is an option to set the CURRENT SCHEMA or CURRENT SQL ID as an parameter under advanced settings in your Configuration Assistant/ODBC data source manager/db2cli.ini .
Also: specifying fully qualified table names in your application might help.