Can you edit a published instructable, Answered

Yep -log in, go to your profile, click "instructables" (it is one of the options in your profile)
Then a list of your published instructables should apear, with a little (edit) by the title :)

After editing a previously published Instructable, is it necessary to re-publish it?

Yes, you can edit published instructables.

If you do that, your followers will get an automatic message to say you have changed something.

What about an instrucrtable that gets edited during q contest ??

A

Yes, as the author you retain complete rights to change things any way you like and that includes deleting it if you so choose to do so.