You can switch between currently opened files and windows in SSMS with Ctrl + Tab and Ctrl + Shift + Tab. The interface can become pretty noisy sometimes when you have too many files open. An alternative way to do this is by clicking on the dropdown arrow at the extreme right of the files tab and seeing a list of all the open files. You can also open the dropdown by pressing Alt + Ctrl + Down keys. If you are looking for a specific file, you can start typing the name of the file, and it should get selected.