To set or change program access password you have to run Email Spy Control Panel.
If program is in visible mode you can click on Email Spy Control Panel shortcut within Windows Start menu (as below):
Start -> Programs -> Email Spy -> Email Spy Control Panel
If Email Spy is in stealth mode you have to press program hotkey to run Control Panel (program default hotkey is Ctrl + Alt + Shift + E).
Then click to Security tab to get following window:

Click to Change and you will get following window:

Here you can set or change Email Spy access password.