How can I thicken this area? I'm trying to 3D print this chair, but the lumbar area is too thin. The rest prints fine. I've meticulously selected the boundary of the area in question (I suspect one of the loop functions would have helped me there). How can I thicken that area to match the upper section of the chair? As you might guess, I'm pretty new to Blender. Thanks.
Just to answer the question, disregarding how the geometry looks:
Try to do a soft selection and go on from there.

So first go into edit mode.

Then enable proportional editing. (press the round dot/circle icon at the bottom. Its the one left of the magnet symbol. End select "Connected")
Now you can select a bunch of faces or vertices and use your scrollwheel. The circle that appears marks the soft selection area.

If you drag your vertices outward (using scale or simple move) the neigbours are dragged along with it.

You can influence the shape you are dragging by selection a different shape from the proportional editing menu.