Underneath the shutter button in the Pilot app, there's a button with a few horizontal sliders as its icon. Tap that and a panel with manual/advanced settings should appear on the right side of the screen. From there, you can control ISO and exposure length.
I've always used ISO 100 for the long exposures at night. My understanding is that anything higher will end up very grainy.
I've been able to get decent results up to 4", myself, but anything over 2" usually requires several attempts before one comes out sharp. On a calm night, I've heard that some folks have managed to get 8" to come out looking nice, but I haven't ever myself.
For me, the key has been to wait until the h/s and v/s movement indicators have read 0.0 for a second or two. The gimbal is so good that the Pilot app's view seems stationary a bit before the P3 truly has stopped moving, but that last bit of drift/momentum will ruin a long exposure. Then, take at least 2-3 shots for every scene you want to photograph. Even at 2", I usually end up deleting half of what I captured due to blurriness caused by slight movements (and you can't always see that until you view them on a larger display, so just take lots to be safe).