How to do Port forwarding from 443 to 80 ?

The problem has nothing to do with port forwarding at all, but is simply trying to patch a limitation in your app that hard-codes links as "HTTP" (absolute urls), when it should use relative URLs, and allow the browser to decide whether it should be HTTP or HTTPS.

Apart from actually fixing the app itself (which may be as simple as flipping a setting somewhere), you may be able to use a "stream" or "rewrite" profile to rewrite all possible links from HTTP to HTTPS, and to make sure that the host part of the links points to the same F5 VIP. Using an irule that redirects HTTP to HTTPS may work too, but that is going to cause POSTS that get a 302 response to be lost.

The best and easiest way is to find a solution in the application itself. Failing that, you're gonna have to play around with the stream and rewrite profile, and irules and hope you catch every case.