For this kind of object you should better work in a non-destructive way, it would allow you to easily change the shape, for example begin with this kind of mesh (mirrored with a Mirror modifier):
Give it an Array modifier:
And at the end give it a Simple Deform in Bend mode:
Continue to model, always keep a non-destructive version of your object so that you can go back if necessary.
You could also use the Object Offset option of the Array although the Simpled Deform is probably more flexible in this case.
For your final shape, extrude up the mesh, remove the inner faces:
If you want the round shape to be part of the same mesh, extrude up, extrude inwards:
Create some loopcuts with CtrlR:
Move them up to have your rounded shape:
If you plan to give it a Subdivision Surface, you'll need to bevel some edges: