Please navigate to your Editor under Customize.
Open the following folders (by clicking the the grey triangles):
In that folder, you will find the expiry-date.twig file. Click on the pencil icon, above the editable text area, replace the Good Through in line 4 with your choice of words. Click the disk icon, which replaced the pencil icon, to save, and hit the green Publish button, once you are done with all your changes. If you have different language versions in your shop, make sure to change the language version in the drop down menu. If you would like to add a language which is not available in drop down menu, click the plus icon.
If you want someone from our support team to have a look at your case specifically, please get in touch with our support at email@example.com and provide your space ID.