The checkout page is very complex, and most of it is automatically generated. You do not have control over fields, or form labels.
One thing you can do, however, is add in blocks (e.g. content blocks) to top / bottom of the page.
IMPORTANT - if you make any changes to this page, run a test transaction to make sure you haven't introduced any errors.
Login to your website - go to checkout page, edit page, add in blocks
Note - if you know HTML, you can style up your div blocks using "callout" class
ADD BLOCK TO CUSTOM CHECKOUT INFO AREA
CHOOSE CONTENT BLOCK
ADD IN CONTENT & SAVE
NOTE - You can also add in custom content to footer areas