Ted here from Shopify, thanks for posting your query.
Sure thing, so you should be able to change the wording within the language editor as shown in this guide. I've added a quick screenshot on how that should look too for convenience.
If there is anything else I can help you with, please let me know, I'm happy to help!
I came across your query and thought of sharing a tip that you may brighten up your store. A simple way of highlighting products with 'Back Soon' or 'Sold Out' tags is through product badges & labels. Here's a screenshot demonstrating the same! You can do this easily via the ModeMagic app which is free to start and has 24/7 customer support. Do check it out and if it meets your store needs, would love to hear about your experience!
I hope this helps!