Now, what if something has gone wrong?
When it comes to functionality, it’s almost invariably a plugin-related issue (WordPress core developers aren’t in the habit of breaking things majorly when it comes to updates). Depending upon the nature of the issue, you may be able to guess the culprit (for example, if your contact form is broken, it’s likely that there’s an issue with your contact form plugin), in which case, you can deactivate it. Otherwise, you should deactivate and reactivate each plugin in turn until you discover the offending plugin.
Once you’ve identified the plugin causing the issue, you have a decision to make: ditch the plugin and find a better alternative, or roll back to a previous backup and contact the developer for further information (they may well have a fix on the way). As a rule of thumb, if you’re looking at a plugin that hasn’t been updated in months, it’s probably best that you move on. Checking out reviews on WordPress.org can also be a good indicator (i.e. are other people having similar issues?).
All of the above may sound like a real headache, but in reality, you’re unlikely to have to take many (if any) of the above steps. The vast majority of the time it will simply be a case of clicking ‘Update’ and moving on with your day. The alternative – not updating core, themes and plugins – is an absolute no-no, for reasons already covered.