Git: how to go back to a previous commit -

i in situation in master:

     --c1--c2--c3--c4 

i in c4, don't it, go c1 , work on in master:

        ______________           /              \     --c1--c2--c3--c4   c6 

please how achieve that?

thanks! :-)

do -

git checkout c1 

return master branch

git checkout master 

git checkout -, checkout previous branch or commit head pointed at.

let me know if helps.

thanks!