I have lightning flow built in way that if Account has Multiple Contact records , if one contact has been choosed as Primay contact (PrimayContact Checkbox field in Contact object) then other records will set as false in Primary contact checkbox field. 

But, I want to display a alert message in UI when they try to change other contact as primay its accepted to change but need to show alert or warning message stating that (" would to like to change the existing primary contact to new one" )something like that. could someone suggest how can i implement this? Below is flow blocks. Thanks.