How to Update ACF Field value using Rest API in wordpress?

I had the same problem and sticked together this javascript-code from various sources, which works great now:

      const username = 'your_wp_username'
      const pass = 'your_wp_appliaction_password'
      const token = window.btoa(`${username}:${pass}`)
      const url = 'https://yoururl.com/wp-json/acf/v3/projects/384?fields[title]=Better title';
      const params = {
        'cache': 'no-cache',
        'method': 'PATCH',
        'credentials': 'omit',
        'headers': {
          'Authorization': `Basic ${token}`,
          'Content-Type': 'application/json; charset=UTF-8; application/x-www-form-urlencoded',
        },
      }
      fetch(url,params)
      .then((res) => {
        return res.json();
      })
      .then((data) => {
        console.log(data)
      })

Make sure to replace the following variables in my code:

your_wp_username
You can find your wordpress-username in: WP-Backend --> User --> Edit

your_wp_application_password
You can generate an Application Specific Password in: WP-Backend --> User --> Edit --> Scroll to the bottom or by following this guide. The password will look something like this: XXXX XXXX XXXX XXXX XXXX XXXX

your_url

• Make sure to change the url https://yoururl.com/wp-json/acf/v3/projects/ to your custom API endpoint.
• The number 384 after the endpoint is the ID of the element / post we're updating in this request
• The word title in the url is the fieldname we're updating