Task: I forgot to add many details about my wife like surname and email id  while saving her contact in my contact list, so edit this contact and mention full name Clara Wagner, add email id clarawagner98b@gmail.com in the Contacts app.
Steps:
Click on the Clara's contact option to view the edit option of this contact.
Click on the edit icon of this contact at the top of the screen to edit this contact.
Click on the Surname input box to set the surname of my wife.
Type Wagner in the surname section to set the surname of this contact.
Scroll down to view the Email section to add an email address to this contact.
Click on the Email section to add the email address to this contact. 
Type clarawagner98@gmail.com in the email section to save an email of this contact.
Click on the right tick icon at the top of the screen to save the changes in this contact.