i noticed on hap lite that cpu frequency can go up to 750mhz.
1 - is it safe to oveclock it?
2 - can this setting damage the router somehow?
3 - will the router overheat if i set freq to 750mhz?

overclocking in general is not an exact science, all devices dont have the same overclocking potential, overclocking always imply the risk of damaging the hardware.

if you are aware of the risk and you want to try, the best way to do it is going step by step and testing device stability torturing it with traffic to get cpu at 100% of usage and see if it remains stable without rebooting or failing

i have tested some hap lite working fine at 750mhz

unless you're in Extremely hot location, power supply quality is poor(but linear PSU bit more noise-proof in that term. but more prone to brownout and overwoltage than AC-DC-DC-based) and there was Extreme vibrations(heatsink is hold at CPU by termal compound, which is Strong, but had some Limitations) you're safe to overclock it.
bottom note: MikroTik puposely limits overclocking to VERY Safe limits. so unless you had very "unluckly" device that can't run even at stock clocks with temps below 40'C, its worth to try.
p.s.
remember me old days where folks OC some(not all support much of) RB's from command line(Winbox didn't support all range) and put crazy fans atop heatsinks to compensate that soldering hype(and profit from modifications)are quite high for some models. i even notice some/few of such units in some companies.(in Feb i just saw 450g for example

thank you for your information.
i will consider to change this setting only if cpu throttles to 100% for long time and check the result, cause i use hap lite to coffee shops where 100+ clients are simultaneously connected. incredible little device with tremendous power can hold too many clients when other devices fail and reboot on 20-25 users. this is where wireless dual chain shines. i have seen up to 120 clients on wireless registration list.

Active cooling modification would help a lot...