If you want to change the number of divisions on a lattice whose points have been moved, choose Edit Deformers > Lattice > Remove Lattice Tweaks, and then change the divisions of the lattice. If you want to change the divisions on a lattice with history, find the input lattice shape and change its divisions. You can find the input lattice by selecting the lattice and looking in the attribute editor tabs for the original lattice shape, which will typically share the same base name as the output lattice appended by “Orig.”