Well, here's the thing. Yes, you can update the template.
I would be very careful if you make more than minor changes. All of your emails will go into draft mode. Then you should go to each draft and click Edit Draft.
if everything is ok, then try a Test or Preview.
If that works, THEN and only then, approve the email.
If it doesn't work then, you need to look:
https://community.marketo.com/MarketoTutorial?id=kA250000000Kz3b
https://community.marketo.com/MarketoDiscussionDetail?id=90650000000PV2NAAW
https://community.marketo.com/MarketoDiscussionDetail?id=90650000000PH46AAG