Upgrading to the latest WordPress version will keep your website secure from any malicious code or content. It also makes your website faster. The newest version of WordPress always comes with additional features and it usually fixes any issues from the previous versions. (google)
First do the upgrade to a clone. May be to be bugs, because you may need to upgrade also the plugins or the Theme, possible not at the same time.
Before the upgade of WP, Plugins and Theme to take a good backup.
In case you have modification of the Theme, you need to do these the child Theme of site.
In case you made modifications in a plugin change the name.
In case you make translation send the translation to the author of the Plugin or Theme.
