How to change field names in SP Login page and "Forgot Password" page?

Modify the message value in sys_ui_message table

Step 1 : Navigate to sys_ui_message

Step 2 : Apply the filter as shown below.

Step 3: Modify the corresponding message value as per your requirement and Save the changes. 

Step 4: Logout and navigate to your login page

Now your login page will be shown with updated messages!!