How Move Around In Tmux

Ctrl-b then [ then you can use your normal navigation keys to scroll around (eg. Up Arrow or PgDn). Press q to quit scroll mode.

Alternatively you can press Ctrl-b PgUp to go directly into copy mode and scroll one page up (which is what it sounds like you will want most of the time)

In vi mode (see below), you can also scroll the page up/down line by line using Shift-k and Shift-j (if you’re already in scroll mode). Unshifted, the cursor moves instead of the page.