The "label" for a field is what displays next to the field and prompts the client to fill in that information. The "hint" displays underneath the field and can be used to provide a bit more information around what you're looking for them to fill in there.
Maybe in your lingo, you ask clients to fill in their "Mobile" number rather than their "Cell" number. If that's the case, it's easy to update the label for where you're capturing that phone number to better match your business. To start just go to Settings → Client Information and click either the "Define Field Labels" link from the left hand menu or the "Add or Remove Fields" link from the Client Info section (they'll both take you to the same place):
On the next screen, find the field that you need to change the label for (in this case we're looking for the cell phone field but it could be a different field for you). Click the Cell Phone field and this will open the Field Setting to the right of the screen:
In the Field Settings go to the Field Label section and change the display name from "Cell Phone" to "Mobile Phone".
You can also add a hint that will display beneath the field and give clients and/or staff a better idea of what to put in the field. In this case, I'm going to type in a hint that tells clients and my fellow staff how I would like the mobile phone to be formatted:
Next, you'll want to click the "Save Changes to Field" button which will commit the changes that you've made to your field label and hint:
You can then click the "Preview Form" button to the right of the "Build Form" button to see how your field will look now to your clients on your scheduler:
Now you should be able to see the field added to your client form: