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Use the wpmem_default_text hook to filter any user facing text:
https://rocketgeek.com/plugins/wp-members/docs/filter-hooks/wpmem_default_text/

In the documentation link above there is an example filter that changes both the login heading (what you asked about) as well as the register heading.

There’s an article here that should help with applying filters:
https://rocketgeek.com/basics/using-code-snippets-from-the-site/