I've just done that on Gentoo source installed nginx by following these simple steps:

* Get old version config flags with -V, e.g /opt/nginx/sbin/nginx -V

Mine had:

nginx: configure arguments: --prefix=/opt/nginx --with-http_ssl_module --with-cc-opt=-Wno-error --add-module=/usr/lib/ruby/gems/1.8/gems/passenger-3.0.9/ext/nginx

Download tarball:

* wget http://nginx.org/download/nginx-1.0.14.tar.gz

Extract and configure passing the same flags:

* tar zxvf nginx-1.0.14.tar.gz && cd nginx-1.0.14

* ./configure --prefix=/opt/nginx --with-http_ssl_module --with-cc-opt=-Wno-error --add-module=/usr/lib/ruby/gems/1.8/gems/passenger-3.0.9/ext/nginx

Compile and install

* make && make install clean

The make install action will check for old directories and files so no need to worry about stuff being overriden.

Verify your nginx version with:

/opt/nginx/sbin/nginx -v

nginx version: nginx/1.0.14

:)