Use this command to remove entries from known_hosts:
ssh-keygen -R hostname
I have only recently started using host key’s, but when I have messed with them it is generally one key per line so backup the file and remove them one at a time until you find the right one. Then add the others back. Bit of a long way to do it, but should work.
Also based on that error, and with no idea what so ever, it could be the first host key in the file that is the problem so open up the file with vim
then save it.