centering an object

how would u alter the center of an object after its already created?

for example if i make a circle and iw ant it to be centered at the top how would i go about doing that?

or to fix an existing mesh’s centerpoint

pivot menu: median
edit mode: select point[s] that you want to be median.
shift+s: snap curser to selection
object mode: editing buttons (f9): mesh panal: center curser

for the objects center just do center new