BuddyPress.org

You have two options to edit the string:

1) Edit the language label (preferred)

The “username” can be modified with a custom language file.

Read the “Labels and Messages” section at the URL below to find out how:

https://codex.buddypress.org/how-to-guides/customizing-labels-messages-and-urls/

I use the program, PoEdit, to make such changes.

2) Edit the template

The login form can be found in /bp-themes/bp-default/sidebar.php.

Copy it over to your child theme and edit accordingly.