We cant do that. You should contact letterbox. The admins control the artwork there. Maybe try via Twitter?

And just to dampen the expectations a bit, the current poster on LB was added by one of the LB founders (also a mod here) so it was likely an official decision to change the poster.

Note that I moved that poster to Hungarian based on no English text and original title only. The English title is without accents.

Can someone please tell me the process of changing posters in letterboxd.

Have you read their FAQ?

 Why does a film on TMDb have a different poster? To maintain a consistent user experience, we lock the posters of popular films from being updated once they are released. If you spot a released film with an incorrect or inferior poster, please let us know and we’ll consider updating it (due to the volume of requests we are not able to respond individually to poster requests). Posters for unreleased films are updated regularly based on TMDb user votes.

You can just use the little report flag on the Letterboxd entry to politely ask them to consider changing the poster. I believe they also prefer high quality title-only posters.