You can move files or folders by selecting the three dot menu next to the document and choosing move to...

In the new pop up menu, you can choose a subfolder already within the current folder the document is sitting in.

If you need to move the file to another folder, you can navigate back through the folders by selecting the folder at the top of the list with the arrow next to it. Keep going until you get to the right folder.

Click move when you've selected the right folder to move the file or folder to.