keywords are not edited after posting.
absolutely not correct statement!
you cant edit the 1st tag (which, in case you've shared your post to community instead of your own personal blog, indeed cant be changed - 1st tag is the tag of a certain community).
in other cases you CAN EDIT ANY TAG of your post.
if you have difficulties with editing... well, what can I say?
there is something you doing wrong, and I cant say what it is, as i am not telepath : )
also, if you used maximum of 10 tags, than you can not add 11-th tag, thats correct, you have to delete one, and to change it to another.