A family of Microsoft word processing software products for creating web, email, and print documents.
Something (maybe an add-in or maybe a permission problem) is preventing Word from writing the value from the Options dialog to the registry entry that should store it permanently.
You can change the registry value directly, and later you can investigate what caused the problem.
First, follow the instructions in https://support.microsoft.com/en-us/kb/322756 to back up at least the key HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Word\Options and possibly the entire Word key. That will create a file that you can use to restore the original data if necessary.
While the key HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Word\Options is selected, the right side of the registry editor shows the list of values that it contains. Double-click the name of the AutosaveInterval value, which opens a small dialog box.
Select the Decimal option button, and type the desired number of minutes into the Value data box. Click OK, verify that the value has changed (the right-hand side shows both the hexadecimal and decimal values), and close the registry editor. Restart Word and verify that the Options dialog shows the new value.
For troubleshooting possible add-in problems, follow http://support.microsoft.com/kb/921541 starting at the heading "Disable the Startup folder add-ins"about halfway down the page.