How to change tags in visual mode?

[twmht]

hi,

here is an example:

<firm>
    <legend>Contace details</legend>
    <label>
        Email:
        <input type = "text" name = "email" /><br />
    </label>
</firm>

I want to change the <firm> into <form>

first, I select the body(multiple lines) surround in <firm> and let vim go to visual mode,

but how can I change <firm> to <form> in visual mode? (similar to cst in normal mode)

Thanks in advance:)