This is a common problem for all the SEO experts that they update or revised their content but Google doesn’t crawl their website on time. Sometimes it crawled but not fetches the revised content. There is no direct rule that how much content should be updated so Google will crawl your content. Suppose you have changed the only date and some words in content then it doesn’t mean that you have changed your content and it will be crawled by Google.
People try to play game with Google and they replace paragraph from top to bottom and bottom to top. Google is wise in this case and knows very well your activities. But suppose if you have written “5 SEO points for on page SEO 2018” and now you want to change it for 2019, and you have also changed the huge section of content then it will be counted as a content update. In other words, content should be different.
Google is behaving himself as real world behave. Suppose if I have sent you an article through WhatsApp and again and again, I am sending the same article then anyone shouldn’t be interested. The same thing is happen with Google.
If your content is changed and you have added some interesting things in it, still Google is not indexing it. Then you should go to Google Search Console and then resubmit the sitemap. Google will crawl your website in the next two to three days. If you have a time-sensitive content then you can choose the fetch as Google option, it will crawl the content in the next few hours. But you should not misuse this option because Google will give you importance then if you will give them.