Use a jewlers screwdriver and find the tab at the front of the cylinder, looking thru the window.
Angle the screwdriver and depress this tab. While holding the tab in, PUT THE KEY BACK IN.
If you depress the worng tab, the key will not go back in.
Once the key goes back in, you can stop depressing the tab, as the key holds it down. If you can, try to look inside the window to confirm that the front tab is down.
Pull the cylinder apart.