The file that you tried to edit is on the read-only System volume and can’t be modified, as you discovered. But you can make the change you want by following these steps.
1. Disable the built-in job to update the locate database
This step is optional, but it will prevent the database from being updated twice in one day, once a week.
sudo launchctl disable system/com.apple.locate
To revert this step:
sudo launchctl enable system/com.apple.locate
2. Create a new job to update the database
Copy the job file to the writable LaunchDaemon folder and give it a different name:
sudo cp -p /System/Library/LaunchDaemons/com.apple.locate.plist /Library/LaunchDaemons/locate.plist
The name shown is just an example; you can use any (unique) name you want. Now edit the new file so that it reads as follows:
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
<key>Label</key>
<string>locate</string>
<key>ProgramArguments</key>
<array>
<string>/usr/libexec/locate.updatedb</string>
</array>
<key>ProcessType</key>
<string>Background</string>
<key>KeepAlive</key>
<dict>
<key>PathState</key>
<dict>
<key>/var/db/locate.database</key>
<false/>
</dict>
</dict>
<key>StartCalendarInterval</key>
<dict>
<key>Hour</key>
<integer>3</integer>
<key>Minute</key>
<integer>15</integer>
</dict>
<key>AbandonProcessGroup</key>
<true/>
</dict>
</plist>
Finally, start the new job:
sudo launchctl enable system/locate
To test the configuration, you can run the job immediately:
sudo launchctl kickstart system/locate