other-window). That is o, not zero.
To select a different window, click with Mouse-1 on its mode
line. With the keyboard, you can switch windows by typing C-x o
other-window). That is an o, for `other', not a zero.
When there are more than two windows, this command moves through all the
windows in a cyclic order, generally top to bottom and left to right.
After the rightmost and bottommost window, it goes back to the one at
the upper left corner. A numeric argument means to move several steps
in the cyclic order of windows. A negative argument moves around the
cycle in the opposite order. When the minibuffer is active, the
minibuffer is the last window in the cycle; you can switch from the
minibuffer window to one of the other windows, and later switch back and
finish supplying the minibuffer argument that is requested.
See section Editing in the Minibuffer.
The usual scrolling commands (see section Controlling the Display) apply to the selected
window only, but there is one command to scroll the next window.
scroll-other-window) scrolls the window that
C-x o would select. It takes arguments, positive and negative,
like C-v. (In the minibuffer, C-M-v scrolls the window
that contains the minibuffer help display, if any, rather than the
next window in the standard cyclic order.)
M-x compare-windows lets you compare two files or
buffers visible in two windows, by moving through them to the next
mismatch. See section Comparing Files, for details.