How to release this connector?!

We all appreciate the pictures, but maybe year model of the truck and which piece this is exactly........ I'm sure a hammer would work also.... but you might need new parts after that.

 

Small flat head screwdriver

 

If that is flex conduit, you can separate the connector with a small flat head screwdriver as @saybng says, but I doubt that will help you unless you only need to replace the conduit. That potted seal on the end of the elbow does not bode well for you.

That "connector" appears to be a connector only in the sense of the flex conduit. There is not likely to be any terminals inside that connector, only wires running through that are likely terminated somewhere inside the component. It's just like running 120V wire through conduit...you can disconnect the conduit at a box, but the wires are terminated on devices inside the box.

 

Evap cannister / vapour lines?

 

I think this is what you need to do:

The tab marked with the red arrow and the one directly on the other side needs to be squeezed together. This should expand the 2 clips marked with the yellow arrows to release the fitting. A bit of twisting and pulling and you should be all set.