1) Click "Subscribers"
2) Click "Forms"
3) Find the sign up form you would like to modify and click the "Edit HTML" link underneath it's name.
this page you can choose to modify your sign up form, the thank you
page, or activation page used if you have enabled double opt-in on the
form. Click the "edit" link to the right of the particular page you
would like to modify.
Click "Custom" to make the HTML for the page visible. Make any changes
you would like to the HTML. Once this is done press "Save Changes" at
the bottom. If at any time you would like to reset the form back to its
original state you can select the "Default" for it to be restored to
the default page.