Regardless of how it happened, it's important to fix broken links so that your visitor does not get confused. To fix the broken links problem first you have to identify what type of broken links are they are.
Most of the time broken links occur due to typos. For example, you wanted a link like http://www.mysite.com/aboutus
but typed like this http://www.mysite.com/about-us
If your one is similar to this issue, please go through the URL again and fix it first.
Sometimes broken links occur due to redirects issues. If you recently making any URL changes to a page, all the links will send the user to a 404 rather than the exact page.
If your site uses Apache you just need to set up redirects in your .htaccess file.
To fix the broken links problem, you can just entirely delete those links or can replace them with valid links.
To avoid such broken links issue in the future must double-check every link. Also can take help from an SEO company
because they have experience in keeping large site architectures without breaking them.