In Microsoft Outlook 2016, Outlook 2013 and Outlook 2010, you are not able to modify the Exchange Server or User Name in the Account Settings of an Exchange account that already exists. In Outlook 2016, the Exchange Server field is not listed in the Change Account dialog box, and in Outlook 2013 and 2010, the Server field is listed but it is not editable. This behavior is by design in these products with existing Exchange accounts.
In Outlook 2010 (version 14.0.7012.1000 and earlier) and earlier versions of Outlook, you could modify the Exchange Server and the User Name fields in the Change Account dialog box if you accessed it from Mail in the Control Panel while Outlook is not running. Starting with version 14.0.7155.5000 (the August 2015 update) of Outlook 2010, this was changed and you can no longer change these settings for an existing Exchange account.
With POP and IMAP accounts, you can modify the accounts mail server settings in any version of Outlook.
Instead of trying to change the Exchange Server in an existing account in Outlook, you should either remove the current account and re-add it, or if you prefer to not remove the account, create a new Outlook profile and setup a new account in the new profile. See the following articles for more information:
Create an Outlook profile in Outlook 2013 or Outlook 2010 – https://support.office.com/en-us/article/Create-an-Outlook-profile-F544C1BA-3352-4B3B-BE0B-8D42A540459D
Outlook email setup – https://support.office.com/en-US/article/Outlook-email-setup-6e27792a-9267-4aa4-8bb6-c84ef146101b