Overclocking/underclocking isn't awfully hard, just for a quick and simple boost in performance you just need to crank up the multiplier on the CPU (If it's unlocked, like in your case) and some voltage and then test for stability using an application like Prime95 or similar CPU stress test. If it succeeds without the computer hard locking itself or errors, or bluescreens, etc you can either keep bumping up the multiplier or lower the voltage if you're already at a comfortable clock speed. Now if you want to squeeze every last bit of performance, there are many other factors that I won't get into as it can get exponentially complex.
In your case, given that you're tight with CPU temps it's exactly the same but in reverse, but in this case you want to prioritize voltage, you want to lower the voltage as much as you can without compromising stability. In your case, taking that the CPU runs at a 1.475v, you can try 1.425v then stress test for about half an hour or so, if succeeds keep lowering the voltage until it fails. If your temps are still too high, then lower the multiplier. Basically rinse and repeat the process.
From experience these APUs will basically make the computer go black screen if unstable (at least on my particular system), I tried to undervolt mine.
Keep in mind that any application that is designed to fully stress your CPU will put an unrealistic load on it, thus temps will be higher than during normal load.
A great cpu heatsink that I've installed in a couple systems that is also quite cheap is the CoolerMaster Hyper 212 Evo.
