You will now want to put the new knob on the rotary encoder. You can see that there are 2 rounded sides and 2 flat sides and they have to go together in that orientation.
Test fit it first and make sure it will fit snugly. You will have to use some pressure to get it on but don't get excessive. You may have to use a razor blade or exacto knife to shave a bit of plastic off of the rounded or flat sides of the inner wall to get a smooth, but snug, fit on the rotary encoder.