I’m using icingaweb2 2.9.5 …
If i want to do maintenance i use icingaweb2 to add a fixed downtime to a host (e.g. 10-11 PM). That works fine. Now what do i do if my maintenance only took 15 minutes and i want icinga to check and notifiy the host again? There doesn’t seem to be a way to update or edit the fixed downtime, i can only delete it.
Deleting feels wrong since i assume that removes it completely, messing up my history and SLA if i try to calulate those?
Am i doing something wrong here or is that option mission for a good purpose i just don’t know? 
you can delete a downtime, the downtime was still active for the time before deletion
click on the service and then on the history tab to see the way the downtime behaves
That is indeed correct. Did not expect that with that button name.
Is there a way to edit downtimes too? F.ex. extend by an hour?
if it is a scheduled downtime created with icinga director, you can edit it, and a deploy will tell icinga2 to do so.
icingaweb2 (monitoring/icingadb) has no possibility to update a downtime, but you can create an new one overlapping the old one and delete the old one.