Is there a way to use update command to locate files on OS X without having to run updatedb manually?
If you want locate to update more frequently, you have to modify the /System/Library/LaunchDaemons/com.apple.locate.plist file.

Specifically, you want to find the &lt;key&gt;StartCalendarInterval&lt;/key&gt; section and modify it to your specifications.  By default, locate is configured to update at 3:15am every Saturday (Sunday being the first day staring with 0).

&lt;key&gt;StartCalendarInterval&lt;/key&gt;
        &lt;dict&gt;
                &lt;key&gt;Hour&lt;/key&gt;
                &lt;integer&gt;3&lt;/integer&gt;
                &lt;key&gt;Minute&lt;/key&gt;
                &lt;integer&gt;15&lt;/integer&gt;
                &lt;key&gt;Weekday&lt;/key&gt;
                &lt;integer&gt;6&lt;/integer&gt;
        &lt;/dict&gt;


This works like cron, so you can specify an asterisk if you want, for example to run it every day.  Just change the 6 to and * and it will run at 3:15 every day.

An easier way to do this if you want to have it run at a set interval, for example, every 3 hours, you can use the StartInterval &lt;integer&gt; directive.  So, for your task to run every three hours insert the following directive into the plist file:

&lt;key StartInterval&lt;/key&gt;
   &lt;integer&gt;108000&lt;/integer&gt;


The integer is the number of seconds.  So, 60 seconds in a minute, 60 minutes in an hour and so forth.

Don't forget to remove the StartCalendarInterval directive.

Last, but definitely not least...make a backup copy of your .plist file before modifying it.  This way, if you hose things up, you have a good config to fall back on.

Note:  In order to make these changes work under OS X El Capitan, you need to disable OS X's System Integrity Protection (SIP).  You can ONLY do that by using csrutil in system Recovery Mode.

Boot Mac in System Recovery by holding Command+R.

Using Terminal, enter csrutil disable

You should see the following:
Successfully disabled System Integrity Protection.  Please restart machine for changes to take effect.

Reboot.

locate should now update per your schedule.

Disclaimer:  This is not a recommended procedure.  While this answer technically does address the question, it doesn't cover the "Why is this necessary to alter the updatedb schedule?" question.