This information initial value is (maybe) stored in Windows Register (register.exe) at Software\SAP\SAPGUI Front\SAP Frontend Server\Filetransfer -> PathDownload, you can read it thru class CL_GUI_FRONTEND_SERVICES method GET_UPLOAD_DOWNLOAD_PATH and update it thru method REGISTRY_SET_VALUE. (Else there may be some parameter ID to force data, but I'm no longer sure)
Regards,
Raymond