Hi Vanessa,
I am not aware that that is a requirement. You are required to display your address, however that will appear automatically once you have filled in the details in Clear Books.
Below is a link to the relevant guidance from HMRC, along with a screenshot of the relevant paragraph:
https://www.gov.uk/invoicing-and-taking-payment-from-customers/invoices-what-they-must-include
https://s3-eu-west-1.amazonaws.com/uploads-eu.hipchat.com/36185/2561547/xQxbKWza98Aynmc/upload.png
If you do want to make changes to the text of the invoice you will need to create a custom invoice template, which will then allow you to edit the text in various fields. Instructions on creating a custom invoice theme can be found in the Support Guide below:
https://www.clearbooks.co.uk/support/guides/invoices-settings-2/invoice-themes-create/
I hope this helps.
All the best.