Problem with progmem memory writting

Good afternoon,

I am starting on the arduino and I'm here with a problem.

I wanted to implement a command "Write Memory", in which, being given a certain "string", type a certain value for a given memory address in flash. I've concluded that for this, I have to use the macro PROGMEM, however I could not find a way, with this macro, to write to the address that is given me. Is there any way to do it?

Greetings and thank you in advance for help

You can't write to flash from a running sketch.