You can't just ask THIS broad of a question and get an answer. I mean, the only way to rank higher is backlinks and good on-page, but I'm sure you already know that. So I assume you're asking for a more detailed answer, which is:
Analyze your on-page factors and make sure you aren't overdoing to keyword stuffing, or if you don't have your keywords present enough.
Scan through your link profile on ahrefs.com and disavow any links that are really irrelevant, belong to blog networks, or have with spun content.
Start building new links on as many platforms as you can and be sure to use hand written content for your contextual links. And don't forget a second tier.
In a nutshell, that is how to "improve in SERP". If you need more details, the search button is on the top left.