How to unclip O2 sensor connector? Can't get it to budge at all?

[doubledown]

Hello,

I have been reading these forums the last few days to try and get my car to pass the new emissions test. I have determined that it is the upstream O2 sensor that needs to be replaced but when I went to replace it I encountered a problem right away with removing the connector. Here are a few pictures.



I have tried pushing in and pulling the black tab on the underside (picture at right) but can't budge it at all? Should I be doing something to the red tab on top at the same time? I tried a search here but nothing came up.

Thanks!