In this blog post, you will learn how to change theme in Magento 2 step by step. Please follow below steps to change your existing theme.
1) Log in to Magento 2 using your admin account.
2) From left sidebar, click Stores, and then Settings -> Configuration.
3) In this page, expand GENERAL section in the panel on the left and choose Design.
4) Now select the theme from list of available themes at Design Theme field.
5) Click on “Save Config”.
6) Clear the cache and then check your theme will be changed.
Thats it. Now you can check new theme on front-side.