I have a long page with a detail of an event. In the right corner of this page is present a floating action button to enter editing mode. Where I can place a back to top button?
Here's something things you could consider
The back to button doesn't need to be floating like the FAB. If there's too many things floating they will be competing for attention. Also putting the back to top at the end of your content ensures that the user has gone through all of it.
You could place the back to top button on the bottom left or bottom centre of this screen.
Hope it helps.
The Floating Action Button should be main feature of the page in your application. There is good practice if you use floating action button specific for the important actions. May I ask please why an user needs to edit the details of an event? Is it for admin panel are you designing or is it for the end user who will just have a look for an event?
I did bit research on when to use FAB. The special thesis from Steve Jones (USER EXPERIENCE IMPLICATIONS OF THE FLOATING ACTION BUTTON) might help you to make reasonable decision.
Furthermore, as per the thesis from Steve Jones, he argues and highlights that FABs are made for creating something new on a specific page. Here, the question of using FAB as edit the action which might contradict the edit guidelines of google material design.
So, FABs are designed for creation/addition of main content of the webpage. You might use FAB for creating new event in your case, but not to edit the event.
Please refer to the this wonderful article Back to Top Button Guidelines.
Bullet points 1, 2, 7 and 8 are worth checking out for answers to your question.
Good luck !