I have a 5 v Arduino promini
But want to run it as 3.3 v
How will I change the clock speed
Since I have read online that the only difference between the both versions is the clock speed of 16 mhz and 8mhz
How do I change it to 8 mhz to run the promini at 3.3v
The Pro Mini's microcontroller is not rated to run at 16 MHz with a 3.3 V supply. This is why the 3.3 V Pro Mini's run at 8 MHz. The external clock source on your 5 V Pro Mini is running at 16 MHz. You could change that to an external 8 MHz clock source, but that will require some fairly advanced soldering skills. The alternative is to configure the microcontroller to use its internal 8 MHz oscillator. You can do that by following these instructions:
That will configure your Pro Mini to use the internal 8 MHz oscillator. It will also install a bootloader that frees up 1.5 kB of precious flash memory on your Pro Mini and allows you to use the watchdog timer. Remember to always use your Pro Mini with the same MiniCore board and clock setting after that. You will no longer be able to use it with the Tools > Board > Arduino Pro or Pro Mini selection.
The disadvantage of using the internal oscillator is that it is much less accurate than the external crystal or ceramic resonator. In a worst case scenario, this could interfere with serial communication. However, I haven't had a lot of problems with the internal oscillator. You always have the option of setting the Tools > Clock option to "16 MHz External" and then doing a Burn Bootloader to go back to using the external clock source again.
Ok thank you
And what if I solder an 8 mhz crystal
Then what steps do I have to take
You would follow the same instructions as above, except select Tools > Clock > 8 MHz external instead of 8 MHz Internal.
With the external 8 MHz, you can just select Tools > Board > Arduino Pro or Pro Mini and Tools > Processor > ATmega328P (3.3V, 8 MHz) instead of installing MiniCore if you prefer. However, I would still recommend using MiniCore since it's very easy to install. Since you have to go to the trouble of burning the bootloader either way, you might as well get the highest quality bootloader available, rather than the outdated, bloated, buggy thing used by the Arduino Pro or Pro Mini board definition.
Ok ok thx