How to change the date format in an email using liquid

Hey Stella, 

Try this:

{{event_properties.${datepropertyname} | date: "%d.%m.%Y" }}

It's the same syntax for custom attributes, just add "|" after the first closing curly bracket and define the format of the date attribute. 

Hi stella_issupow 

To add to Arso's  great answer here is a reference for time formatting conventions:
strftime reference