A no-brainer (and I should know)
On a jammer, changing the key is of “total no-brainer” class of challenge; if you are using the middle-finger-on-root-note fingering I recommend here.
Fortunately, the musical score is also simple, as the jammer layout provides a automatic translation feature. One picture is worth a whole bunch of words as you can see here:
Like all good designs, you can see the action is simpler than the words needed to explain it; just count the sharps or flats in the score, divide this by 2, giving a shift factor. Just move right or left respectively from C on the root row by this factor, and if you are stuck between keys, move up to the row above, to what I call the dominant line.
If you have a key change, just do as the notes say; you‘ll find the bridging notes of the arrangement naturally “walks” your hand into the new key. Just go with it.
Additional trick: if you are playing in some key that is too far away from the center for convenience, D-flat or C-Sharp for example, just flatten or sharp the key board by a semi-tone, using the shift key provided at the side. This works well.