I want the portion of the bottle as shown by red arrows in the image. So I need to do something like extrusion but how to extrude it towards the center and away from the center?
You can extrude along vertex normals to do that. AltE then 'Region (vertex normals)'
edit if previous version does not has this extrusion mode:
- Remove the ring, for instance CtrlR then X
- Extrude and scale and only on X and Y: E (extrude) s (scale) and ShiftZ (only along X and Y)
- Then CtrlE, bridge the loop