As mentioned above, your options are to change the pots in the original paddles (swap out the 1 MOhm with 10 kOhm) or modify the board: https://armchairarcade.com/perspectives/2019/10/15/hardware-mod-to-support-standard-paddles-on-atari-flashback-9-and-9-gold/
I have done both, and the paddle modification was much easier (gave that FB9 to my sister). Go for that if you can get original paddles easily, and buy decent potentiometers (I gave a link to suitable ones in an earlier post — they have to survive a lot of rotation, so cheap Chinese stuff is not a good idea). Otherwise, unless you are good with soldering tiny resistors, find someone locally who works on cellphone repair or such and can easily do the work. My suggestion is to use 430 kOhm resistors as the 510 kOhm (suggested in the guide linked above) didn’t work well in my FB9. I haven’t swapped them out yet, because mental health is important.