I can't see a way to do so.
This says I should be able to do it, but I never see the More-->Settings. I just see More-->Date & Time.
++++++++++++++++
https://support.google.com/clock/faq/6273949?hl=en
Change timer settings
Open your phone's Clock app .
Tap More MoreAnd then Settings.
Under "Timers":
Pick the ringtone that plays when the timer ends: Tap Timer sound.
This says I should be able to do it, but I never see the More-->Settings. I just see More-->Date & Time.
++++++++++++++++
https://support.google.com/clock/faq/6273949?hl=en
Change timer settings
Open your phone's Clock app .
Tap More MoreAnd then Settings.
Under "Timers":
Pick the ringtone that plays when the timer ends: Tap Timer sound.