How to scroll in the Terminal app "top"?

I have only one suggestion: use htop instead.

sudo apt-get install htop

The point of the "TOP" program is to tell you which processes are using the most resources or are at the TOP of the list.

For a full process list use "ps":

ps aux

You can filter it with "grep":

ps aux | grep firefox

Indeed, the up and down arrow keys will scroll through top on newer versions, specifically at least on the version of top that comes with Ubuntu 16.04.2 (procps-ng version 3.3.10).

But if the arrow keys don't work to scroll for you, it could just be an issue with your terminal emulator.

Take a look at the top man page:

Lastly, some of top's screens or functions require the use of cursor motion keys like the standard arrow keys plus the Home, End, PgUp and PgDn keys. If your terminal or emulator does not provide those keys, the following combinations are accepted as alternatives:

          key      equivalent-key-combinations
          Up       alt + \      or  alt + k
          Down     alt + /      or  alt + j
          Left     alt + <      or  alt + h
          Right    alt + >      or  alt + l (lower case L)
          PgUp     alt + Up     or  alt + ctrl + k
          PgDn     alt + Down   or  alt + ctrl + j
          Home     alt + Left   or  alt + ctrl + h
          End      alt + Right  or  alt + ctrl + l

So if the up and down arrow keys don't work for you (make sure you hold them down, as deto said), try pressing alt + \, alt + /, alt + k and alt + j.

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Note that this section of the man page does not exist on my RHEL 6.6 machine, which has procps-ng version 3.2.8 (you can see the version number with top -v), and so that version of top does not appear have any scrolling capability.