I want to update my feed just once a day. For example: I want to update the feed every day at 12:00 pm. How to do this without a plugin ?
The RSS feed is "updated" every time you post something new. If you want it to always be updated at 12pm then schedule your posts to always be published at 12pm.
You can't. WordPress doesn't "update the feed", it builds the feed in real-time whenever it's requested, much like the web page itself is built upon request. There's no "feed" anywhere for it to update, it's built on demand.