You can easily change the From address in an ServiceNow Email Client Template but changing the Reply to is less obvious. The answer is buried deep down in this question in the community.
In the Email Client Template record, add the Reply to field under the Sender Configuration tab (why this field is not on the form by default is beyond me)
In the Reply to field use the usual syntax:
javascript:current.reply_to="myaddress@mydomain.com"