It's important not to get confused between buffers, windows, and frames in Emacs.
Frames are the Emacs terminology for what are known as "windows" in the operating system (also, if you're running Emacs in a terminal, the entire display of Emacs in that terminal is also considered a frame). A single frame can contain one or more Emacs windows (which divide the frame horizontally or vertically). Each Emacs window can and must contain a view on to one single buffer at a time (though you can choose which buffer an Emacs window shows, and multiple windows can have views on to the same buffer).
It sounds like you have two buffers displayed in two different Emacs windows. It's not clear from your description whether both of those windows are in the same frame or not.
If both windows are in the same frame and point is in the window you wish to keep, you can delete the other window using the
delete-other-windows command. If point is in the window you wish to delete, you can delete it using the
If the two windows are on separate frames and point is in the frame you wish to keep, you can delete the other frame using the
delete-other-frames command. If point is in the frame you wish to delete, you can delete it using the
If you don't actually want to delete a window (or a frame) and would rather simply replace one of the buffers in one of the windows with another buffer, you can move point to the window containing that buffer and then use the
switch-to-buffer command (this command will also create a new buffer if it doesn't already exist).
Note that deleting a window (or a frame) will not delete any buffer. The buffers that they showed will remain open in Emacs, and you can show them inside an existing window by using