Change "required" to asterisk?

Yes, but to change the style of your CF7 forms you would need to edit the CSS style sheets used by your WordPress theme.

See Styling Contact Form for a general explanation of styling CF7 forms using CSS.

There is a link at the bottom of the page to a comprehensive and detailed article on Styling Contact Form 7 Forms. The article shows people, with suitable HTML & CSS skills, how to change the appearance of their Contact Form 7 Forms to meet their particular requirements.

If you find you need more detailed advice, after reading and working through the above links, please include a link to your Contact Form 7 form, so others here can examine your form in detail using Firebug or Chrome Dev Tools to understand the CSS used for your CF7 form elements and provide you with a possible solution.

The word (Required) is just written text. It is possible to type the asterisk manually. However, you can style this also. For instance using CSS:

.required:after{
   content:'*';
  color:#f00;
}

or SASS:

.required {
   &:after{
    content:'*';
    color:#f00;
   }
}

For the guidance, you can for instance put the following in the Contact Form editor:

<p>Please note that fields with an asterisk (<label class="required"></label>) are required fields.</p>

And to use it in the form itself, you could use for instance:

<label class="required">Your Name</label> for required field labels and <label class="anything_but_required">Your Name</label> for the optional fields.

This makes it consistent throughout.

@favdv – that’s a great tip!