At times WordPress has new features. With these new features your plugin developers will start moving on to the new version of WordPress. What this means is that you won't be able to get new features for your plugins or WordPress itself unless you continue to upgrade.
When you do an upgrade, it's generally under Tools > Upgrade and is just a matter of following the steps. If you have an extremely old version of WordPress prior to version 2.0, you may have to export the blog and import it into a newer version of the software since so much has been changed since then.
It's important that you manually backup your database and WordPress files first. You can get to know about 504 gateway timeout nginx, and DNS_PROBE_FINISHED_NXDOMAIN Error.