When you add an email as task, you get a task creation page with something like this as the name
"[[gmail=13e246b7c9734396@gmail, When I add an email as a task how do I rename it?]] "
You can simply edit the part after gmail, to whatever you would like. Instead of "When I add email as a task how do I rename it?" you can change it to "task1".
If you have already added it, if you go to Todoist.com or any of our apps, you can simply hit the gear icon for that task, and click "edit task".
Hope that explains it for you.