For a bottom up cellular shade, if the shade either is not level or if it does not fit tightly to the headrail when the shade is up there is a way to reset the cordless mechanism. To reset the mechanism, bring the shade all the way down to where it can not go down any further. This resets the mechanism, and from here you should be able to lift the shade evenly, and it should be level.
For a Top Down Bottom Up cellular shade, the same is true, except now, you can do that with the bottom rail, and also the top-shade rail that moves. You can pull that top-shade-rail to its lowest level to reset the cordless mechanism, which will level the shade.