How can i undo the effect of 301 redirect?
This is difficult to reverse from the server side, because the effect of this directive is cached on the client side. Many browsers will cache this directive permanently, or at least until the browser cache is cleared somehow.
There is a good Stack Overflow article about 301 redirect caching behavior that includes some helpful insight about how you can use 'Cache-Control' and/or 'Expires' HTTP headers to modify this behavior. However, clients that have already received a 301 redirect in the absence of these modifiers would not be affected.
You could try to put a redirect on the new page, which points back to the first page, but the comments on the Stack Overflow article I mentioned seem to cast doubt on the efficacy of this solution.