Cannot do it directly in the MSDB folder or the package store(the File System Folder) in SSMS/SSIS. Have to export it first. Change it in BIDS, then re-import it back to SQL Server/SSIS/MSDB.
Note: If the exported package file is double-clicked, the Execute Package Utility is opened. But it's hard to fix/change the connection managers, control flow/data flow tasks on this interface. It's better to edit it in BIDS.
