hi,
> Can it be reversed so count from backwards and how can that be done?. And what is U2 used for?. What is Count In?
The circuit uses a 7490 decade counter, this counter can only count 'up' that is '0' thru '9' then back to '0' ,,, repeating
There are counters which can count 'up' and 'down', but they are not pin compatible with 7490. Examples, 74190, 74191..
U2 [7447] in the circuit decodes the 4 bit output from the 7490 counter in order to drive the LED segments.
The U2 [7447] ic would also work with a up/down counter input.
>> What is Count In?
Count in; is the source of the pulses you are counting, it could be a switch or a logic signal.
Does this help?