As of now, it is not possible to update the Sign-up text displayed on the iframe as this is a part of an HTML source and is something which cannot be controlled by Punchh Dashboard.
Please refer the image below of the text referred here:
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This additional disclaimer will be removed upon formal review of this article. The standard Punchh Inc. KB Disclaimer still applies, and can be found at: https://support.punchh.com/hc/en-us/articles/360040100273-Punchh-Inc-Knowledge-Base-Disclaimer