Hi again,
Well, you have counters wired in two basically different arrangements:
One set has the R0 pins wired directly to ground, and the other set has their R0 pins wired to two counter outputs.
What this means is that for the ones wired directly to ground, all you have to do is disconnect one pin and connect it to a push button that has a normally low output, and when you push it it takes the signal high, thus clearing the counter to zero.
For the ones wired to the counter outputs however, you'll need to add at least one OR gate between one output and its R0 input, and connect it to that same push button.
So you have one simple wiring for the counters that have their R0 go to ground, and one slightly more complex wiring that have their R0 go to outputs. Luckily, you only need to break one of the output connections not both in order to add this functionality.
It might look a little strange to have an all zero's count being displayed if you ever intend to make this a 12 hour clock with am/pm instead of a 24 hour military time clock.