undoing something

I made a mistake. Big deal, now that I know that git storage is immutable and changes just add to what's already there.

A quick look at
	git reflog show topic
tells me that {1} is the one I want.  So, how do I rewind branch topic
to point to topic@{1} ?
I did it by making a tag, and then in gitk pointing to it and picking
"reset to here" from the context menu.  

A while back I was looking for the right/easy way to simply repoint my branch to where I wanted. The best answer was to use push. But, I wonder if gitk and other high-level commands simply call a more primitive command that does this? And can I call it directly? Or is push with a dot as the repository just doing this with no excessive processing and trust that the side-effects are all vacuous?

--John (please excuse the footer; it's not my choice)

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