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You can't do it via XOOPS inteface. You should enter the database and change all them in a single query. If you don't know how to edit the database directly, you should meet PHPmyadmin, a free database administration tool.
When you modify anything in PHPmyadmin, it doesn't only do it, but it shows in screen the SQL query, the one that you should need to use if you wanted to do the same change by writing a program (that is, you learn the SQL syntax while you use PHPmyadmin.
You can try to change one user's email, just for test purposes, then put the SQL query here (skip the database name), we'll show you how to write the proper query to make the change you want.