Override default password nag in WordPress

The function is hooked to an action, so you should be able to remove it.

remove_action('admin_notices','default_password_nag');

You can then add back a function of your own choosing. Rename your function to something that won’t conflict with the Core function name and…

add_action('admin_notices','my_password_nag');

You will need to create a plugin or mu-plugin to do it, but those are relatively easy to create.