Useful e.g. when there are 3 windows (0,1,2), you close window#1 so now there are windows #0 and #2, and you wish to make the remaining windows have contiguous indices.

• Run <C-b> . within the window to be renumbered, and enter the new index for the window.
• Equivalently, run :move-window -t <N> where <N> is the target index.
• To swap the indices of two existing windows with indices <A> and <B>, run :swap-window -s <A> -t <B>.

References:

• https://superuser.com/questions/343572/how-do-i-reorder-tmux-windows