I leased a plane - the same as the previous 5 but a day later once I had enough case to pay the deposit. But now I find it has no inventory/no seats. I searched the whole menu for a point where I could take care of my aircrafts inventory but didn't find any.
OK, I figured it out in the meantime. It is not solved in a bad or uncomfortable way but it is well-hidden :-)
In case anyone else has a similar problem I hope the screenshot helps.
Go to fleet management, tick the box to the left of the plane(s) of interest and then a magic box pops up below the list of planes where you can do some fancy stuff like change the seating...