Determine conditions for Rules when changing field value I've a Rule with below specification

Events:
After updating an existing user account

Conditions:
???

Actions
Create a Message

But I don't know how can I set a condition on field in rule condition part that return true when the field changed,any body know how can I add condition on specified field that return true when the field changed?
You need to create the rule for Before saving content rather than After saving content.

Before saving content has both the original node values and the new updated values which allows you to compare the values. 

After saving content only has the updated values, so you won't be able to see what the old values are.