• Download BuddyPress Username changer
• Unzip it .
• Upload bp-username-changer to wp-content/plugin
• Login to Dashboard
• If you are using WordPress Standard, Go to Plugins->Installed and activate Buddypress Username Changer, If you are using WordPress Multisite, go to NetworkAdmin->Plugins and Activate the plugin.
• Now visit  your BudyPress profile->settings and you will see the Change Username option.