github question

[AndyC via Digitalmars-d]

I'm new to git and github, I read the "10 tips" posted earlier, 
and I dont understand the last one: Avoid thrashing.

The story says, "Instead, remove the offending commits, and force 
push your modified branch".

How would one do that?  Is there doc's around that might explain 
that in a little more detail?

Thanks,

-Andy