This is the same command that you would use to go to different windows in Vim (for example, in a fast or various window), everything is explained in
:help windows.txt
But the answer to your question directly:
- CTRL w CTRL h to move to the left window
- CTRL w CTRL l to move to the desired window
then Enter to select the file you want to open.
You might want to read :help netrw
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