How to reduce LED flash rate (frequency)
The fact that there's multiple operating modes indicates that HH5K1 is not in fact a simple device, but most probably a cheap microcontroller¹ or dedicated ASIC that does the blinking. 
I'm afraid that means you can't swap anything to do what you want.

So, build a flasher from scratch that does what you want. I rarely do that, but here, using a NE555 might be the simplest thing to do; there's plenty of online circuit example that you can refer to, but frankly, the application examples in the datasheet(s) do contain astable multivibrators. Add a smallish capacitor and an BJT to the output – and tadah!, you've got a flasher.

Another option is going the same route as the manufacturer of the board you're looking at: Get some logic to do your work; a 0.30€ attiny would absolutely do the job. 


¹ The 51 in the name might hint it's a 8051 core clone, but that's not going to help anyone – that thing, if programmable, is very likely only programmable once.