Chris from Shopify here. You can change, or upload, a logo by using the Customize Theme editor. If you go to Online Store > Themes > Customize Theme, click on Header from the new option panel on the left side. Then, you will see a box that will allow you to upload an image as your logo, or to input text below that box as your logo. Hope that helps, please feel free to contact us at email@example.com for further assistance with this.
Thank you for your question.
Please share your store URL, page URL and also password (if your store has one), I'll help you out to the best of my ability.