how to replace ñ with n in local AD before initiating sync in AAD connect

The IDFix tool is an easy way to do this: https://support.office.com/en-us/article/Install-and-run-the-Office-365-IdFix-tool-f4bd2439-3e41-4169-99f6-3fabdfa326ac?ui=en-US&rs=en-US&ad=US

In general, it depends on the attribute though. For more complex scenarios, you can use PowerShell or tools such as ADModify: https://admodify.codeplex.com/