CTRL-Y

CTRL-Y will scoll up one line
CTRL-E will scroll one line down (stands for extra line)
CTRL-D will scroll down half a page
CTRL-U will scroll up half a page
CTRL-F will scroll one page forward
CTRL-B will scroll one page back

All commands take account, for example to scroll up 5 lines do 5CTRL-Y

Further Reading

:h CTRL-Y :h CTRL-E :h CTRL-D :h CTRL-U :h CTRL-F :h CTRL-B

More Gifs