Translation title

I think you mean that you’d like to change the text (not translate, as it is already translated).

You can change the login form heading with the wpmem_inc_login_args filter:
https://rocketgeek.com/plugins/wp-members/docs/filter-hooks/wpmem_inc_login_args/

If your objective is to change the text, use the wpmem_inc_login_args filter. That’s the correct way to customize output text. Translation files (.po & .mo) get overwritten in updates unless you maintain a custom version somewhere else and then create a filter to load the custom translation files. It’s not a good way to simply change text.