6d and AV Mode

Two questions, what is best mode for shooting video in movie mode on 6d and if shooting video in AV mode can you increase/decrease f/stop and if so how? 6d for dummies says you can but for the life of me I can not figure it out

I have never used anything but manual mode for video, hard for me to imagine letting the camera make those decisions.

Pretty sure you would use the same controls to select the settings for video in Av as you would while shooting stills. But again, I haven't tried it. I'm prepared to be proven wrong.

If you are talking about changing while shooting, you will need a cine lens and do it manually.

I don't do a lot of video, but I find manual mode to be the easiest for the little bit I do.

I don't have any 6D, but the 1DX applies the same principle of automatic exposure for video regardless of whether you set it to Tv, Av or P. It's only in M there's a difference.
As far as I can remember, at least. I very rarely use it for video.