How to filter github pull request by updated date using graphql?

Search can be used to filter pull requests in a range.

Here's an example

{
  search(first: 100, query: "repo:firstcontributions/first-contributions is:pr is:open updated:2020-01-01..2020-07-31", type: ISSUE) {
    nodes {
      ... on PullRequest {
        title
        url
      }
    }
  }
}

This fetches open pull requests between 1st January 2020 and 31st July 2020.

You can see that the search query uses the same syntax. So, basically you can try this by GitHub search too

https://github.com/search?q=repo%3Afirstcontributions%2Ffirst-contributions+is%3Apr+updated%3A2020-01-01..2020-07-31

Documentation about search can be found in GitHub search docs