You can use this CSS to change "Login" field label to "Username"

.mh-popup-login-field--login .mh-register-login-field__label:after,
.mh-register-field--login .mh-register-login-field__label:after {
content: 'Username';
background: #fff!important; padding-right: 40px!important;
display: block;
top:0;
position: absolute;
}

.mh-register-login-field__label {
position:relative!important;
}

.mh-login-register-panel__inner > form .input-group--prepend-icon:first-child > label:before {
content: 'Username'!important;
display: block!important;
}

.mh-login-register-panel__inner > form .input-group--prepend-icon:first-child .input-group__messages {
display:none!important;
}

In this article you can find information how to add custom CSS to any WordPress Theme: How to add CSS code that is future update compatible