You need to handle each case or error manually or separately. SImply click on each error and check it out why error being showing there and find out their reasons and rectify those errors.
Sometimes, you change or edit or remove your web pages or their URL so that is why
404 page not found error generate.
Unlike, if you hyperlink link any broken link with any piece of text inside your content so also this error comes there. So first check it out why errors are showing there, you can find out error generating reasons by clicking on each URL under
crawl errors section.
Hope that will you!