Ash here from the Shopify team. Thank you for posting your question in the Shopify Community!
It sounds like you're trying to change the theme you're using for your online store. This is absolutely possible! While a Shopify account can have up to 20 themes added to it, only one theme can be published at a time. The published theme is referred to in the admin as your Current Theme.
If you want to change which theme is published on your shop, head over to the Online Store> Themes section of the admin, and locate the theme you want to publish. On that theme click Actions> Publish. We have some more information on managing the themes in your account in our Help Center page titled Managing Themes.
Let me know if this helps, and feel free to reply here if you still have any further questions regarding changing the published theme on your account.