On your screen press Ctrl + a + [ to enter Copy Mode, then scroll up/down using keys j or k. Below are some other navigation keys:

h – Move the cursor left by one character
j – Move the cursor down by one line
k – Move the cursor up by one line
l – Move the cursor right by one character
0 – Move to the beginning of the current line
$ – Move to the end of the current line.
G – Moves to the specified line
(defaults to the end of the buffer).
C-u – Scrolls a half page up.
C-b – Scrolls a full page up.
C-d – Scrolls a half page down.
C-f – Scrolls the full page down.