An alternative solution that does not need any modifications to the original file is
<job-scheduling-data xmlns="http://quartznet.sourceforge.net/JobSchedulingData" xmlns:xdt="http://schemas.microsoft.com/XML-Document-Transform">
<schedule>
<trigger>
<cron xdt:Locator="XPath(//*[local-name()='job-scheduling-data']
/*[local-name()='schedule']
/*[local-name()='trigger']
/*[local-name()='cron']
[*[local-name() = 'name'] = 'MyTriggerName'])">
or this syntax...
<cron xdt:Locator="XPath(//*[local-name()='job-scheduling-data' and namespace-uri()='http://quartznet.sourceforge.net/JobSchedulingData']
/*[local-name()='schedule' and namespace-uri()='http://quartznet.sourceforge.net/JobSchedulingData']
/*[local-name()='trigger' and namespace-uri()='http://quartznet.sourceforge.net/JobSchedulingData']
/*[local-name()='cron' and namespace-uri()='http://quartznet.sourceforge.net/JobSchedulingData']
[*[local-name() = 'name'] = 'MyTriggerName'])">
<cron-expression xdt:Transform="Replace">***some data***</cron-expression>
</cron>
</trigger>
</schedule>
see also I am trying to get the Xpath for a value but getting error for nested condition