You can change the labels for any standard form, label changes will reflect in the Instant Builder along with any Form Code.
1. Go to the "edit" screen of your campaign
2. Click on "Opt-In Channels" on the left side column
3. Click on the field you would like to edit, it will turn white and you can than enter your custom text.
4. Click "Save and Exit" and be sure to open the Instant Builder for changes to take effect.