Importing Development Sandboxes to UAT (Unified Sandbox)

Summary

Importing a sandbox made in development to the testing environment so I don't have to remake the changes in UAT.

Content

We just recently moved to 19D and I've been trying to understand how Unified Sandboxes work. I can't seem to find a way to import a downloaded sandbox in the new sandbox app. 

For example: I had to make a few configuration changes in our Development environment and downloaded the sandbox before I published it. Then I went to go make the same exact changes in our Test environment. Before the 19D update, I would import the downloaded sandbox, and just publish the imported sandbox. However, I cannot seem to find this option.