In your Shopify admin panel, select Apps > TeleCheckout.
Scroll to the bottom where you will see a section called Internationalisation.
Click on Edit the Translations or Customise the Texts,
Select the language of the text you would like to edit.
Search the original text using F3 or CTRL+F key, enter your custom checkout text on the right hand side.
On the same page, click the Preview link at the top.
Alternatively You can also add text in a custom Footer on the checkout in Shopify.