How can one do teshuva for speaking Lashon Harah about all Jews, or about all the Jews of a particular city?
A person that spoke lashon hora about will have a hard time asking asking the whole city for mechila. However the bein adom lamakom part can be corrected, with the regular teshuva process. Additionally a tikun for the aveiro would be to do the opposite of what was done and to speak positively about the people of that city. This in a way will undo some of the damage tt was done.
Kesiva v’chasima tova