Changing your mind after you decline a job offer

HI,

I was offered a job at one of my dream companies. Due to complicated reasons, I declined the offer. I know it's considered highly unethical to change your mind after you accept an offer, but what about changing your mind after you decline an offer? Does anyone have any personal experience (either in the hiring or employee position)? Assuming the position is still open, what would be the best way to go about doing this?Thanks, this is really appreciated.

Please help.

I did not find the right solution from the internet.

References:
https://arstechnica.com/civis/viewtopic.php?t=137130
Video Animation Production Agency

Thanks!