Okay, doesn't really change the answer.
i5-8265u, 15W 1.6Ghz base clock, 3.9Ghz max boost.
Basically they are saying that under full load on all cores it is guaranteed to go at least 1.6Ghz unless something is seriously wrong. Under normal circumstances, given adequate cooling, it will always run faster than that under load. It is completely automatic. You'll probably only see 3.9Ghz under single threaded loads, and it may not sustain that for long.
Best thing to do for laptops to improve performance is to take the laptop apart and apply high quality thermal compound and pads to replace the factory ones. (assuming you don't mind replacing it if you mess something up doing that) Cooling pads/laptop docks to keep air flowing through the system.
If you wanted to overclock a laptop, you should have gone for a larger one with an H class processor. They tend to have more wiggle room due to larger heatsinks, and the HK chips have unlocked multipliers.
The BIOS will be very limited in options. You can try software overclocking, but like I said, as soon as the temperatures go up, the CPU will throttle down.