I found out that removing low quality post will be helpful for my site. But this means I will have 404 pages. What is the best thing to do with these page? redirect to high quality content? How will I tell Google that I already removed those pages. Thanks!
You need to replace the content from low to high quality & submit a request in Google webmaster tool.
yes you can just replace low quality content with fresh and unique content and fetch with google webmaster tool.
By deleting low and old quality blog posts and pages, you will definitely increase the average quality of your website. 404 errors indicate that the page is “not found”, which means that the deleted content can stay indexed longer on Google and other SERPs. 410 errors, however, indicate that a blog post has been permanently deleted and informs Google that you have no intention of bringing it back (if you are using WordPress, I recommend the “410 for WordPress” plugin). When deleting your old content, double-check if it’s being properly redirected to a freshly updated article. If you cannot redirect your old blog posts, simply delete them by using a “410 Gone” response code. This way your user experience won’t suffer.