There are two options when you need to make changes to many accounts at the same time and which one you choose depends on what you need to accomplish.
The file-based bulk upload option is the more flexible and powerful option and will allow you to change just about anything, but here we will talk about the modify details search action which in many cases will be quicker and easier
This function allows you to change one or more account attributes to a new value for as many account as you select from the account list or from search results.
What you can't change this way:
Scenario: You have recorded telephone numbers for all users, but you only need them for staff users so want to remove the data from all other personal accounts. Staff users are on a different email domain from other users.
-staff.yourdomain.netin the email field to return all other accounts (the minus at the beginning means NOT)
Scenario: you want to replace user groups with a choice attribute.