You can find the form-login.php in your theme folder:
/wp-content/themes/YOUR-THEME/woocommerce/myaccount/form-login.php
Or in your woocommerce folder:
/wp-content/plugins/woocommerce/templates/myaccount/form-login.php
PS: It's always suggested to copy your woocommerce templates folder to your theme to avoiding overwriting on woocommerce versione update ;)