A quick and dirty way of doing it, assuming the currently active pane is the one you want to copy text from:

1. Enable pane zoom (<prefix> z)

2. Enter copy mode (<prefix> [)

3. Select and copy text

You can also look into the tmux-yank [0] plugin, which copies to the native system clipboard.

[0] https://github.com/tmux-plugins/tmux-yank