Connected Caldera Forms are sequences of forms that are combined to make one form with partial form submission tracking that can use conditions to change which section of the sequence comes next. The previous and next buttons are outputted at the bottom of your form automatically.
To change the label of the next page button:
To change the label of the previous page button:
For both filters, the variable
$form is the configuration of the current form being displayed in the sequence. Therefore you can use
$field['ID'] to change the label of a button for one part of the sequence only.
Where Does This Code Go?
When using WordPress hooks to customize Caldera Forms or other plugins you should not modify the plugin files, or you will lose your changes when you update the plugin. Instead you should create a small plugin to hold the custom code. It's easy, learn how here.
Technically you can add the custom code to your theme's functions.php, but then you will not be able to change your theme and keep these customizations.