"Apache Port Change" chapter of "Advanced Administration Guide" from Plesk documentation should get you started.
It's not just a simple setting which can be enabled though - requires quite a bit to reconfigure/check.
update: it would help if you let us know what exactly is the problem now? which step are you stuck on?
- nginx installation
- changing HTTP/HTTPS ports which are generated by Plesk using websrvmng, i.e. moving Apache to the back-end - described in the document
- nginx configuration and setting up event handlers in Plesk - see examples in the document
this blog post might be useful as well, without part "3. Configuring Apache" - because this can now be done automatically in Plesk 9.3.