How to change the emial ID

If you are an admin user, you can do it via Admin Console->Users(from the left). Click on the user you want to update and change the email id. 

You can also use a!updateUserProfile() function in saveinto of any component

https://docs.appian.com/suite/help/23.2/Update_User_Profile_Smart_Service.html

My doubt is how to change the email address of "abc@gmail.com" to "xyz@gmail.com"?

Please explain what email address you are talking about. And what exactly do you want to achieve?

Is it the Appian email ID, or the email ID associated with the community account? 

It's a generic question not appian email id

abc@gmail.com I would like to replace abc with xyz 

You forgot to change the link color to white. 

well if you want to manipulate value from email string "abc@gmail.com" to "xyz@gmail.com",below code might help you

a!localVariables(
local!email:"abc@gmail.com",
local!sustring:"xyz",
local!newEmail:replace(local!email,1,find("@",local!email)-1,local!sustring),
local!newEmail
)