Cannot access wp-login.php via FTP

Hello,
like a lot of people I’m having issues with the error message

“How do I solve the Headers already sent warning problem?”

I already changed my php version back to 5.3, so my website is all back to normal. Unfortunately I am unable to access my wordpress account/dashboard no more.

When I try to troubleshoot it, in the documentation it says to:

1. Download the file mentioned in the error message via FTP or the file manager provided in your host’s control panel.

Unfortunately the first 3 error messages deal with errors in the wp-login.php file and I cannot seem to locate this file anywhere on my server.

How can I fix this bug, or where do I find the location of this needed file?

Any help is greatl appreciated!

Thank you,
Feli