Display a specific level so you can then edit the Subdiv components.

Press on the subdivision surface and choose Display Level, then the level you want to edit.

or

In the Attribute Editor for the subdivision surface (the shape node), open the Subdiv Component Display section and set the Level attribute.

Edit the next finer level, and refine the surface by one level if a finer level does not already exist.

Press on the subdivision surface and choose Refine Selected.

or

Select components on the subdivision surface and press + .

or

Choose Subdiv Surfaces > Refine Selected Components.

Edit the next coarser level, by selecting the related subdiv component one level higher in the subdiv level hierarchy.

Press on the subdivision surface and choose Select Coarser.

or

Choose Subdiv Surfaces > Select Coarser Components.

or

Select components on the subdivision surface and press + .