My question is, where can i change the admin Message, if a user changes his Event Registration.
The only Message i get is the amount... but if it is a Group Registration, and the user adds a new member, i need the name, address and else.
Where can i change it?
You'll see the admin email being constructed here. Using email tags, you can setup the email as needed here. It will be the same for both individual and group registrations... unless you add additional code to differentiate the two.