Hi Vilishost, and welcome.
TechnoBear has told you how to change the crawl rate. But doing that won't necessarily mean that Google's search results will reflect changes to your site any faster. Regardless of how often Google crawls the site, there can still be a delay before it's reflected in the results. However, a faster crawl will generally mean that entirely new pages show up sooner.
The main reason to change the crawl rate is if you're worried that the crawler is using too much bandwidth.
Also, robots.txt isn't really relevant here. You can use robots.txt to prevent pages being crawled, but as far as I know you can't use it to change the crawl rate.