change ftpd password viassh pure-ftpd
You need to use the following:
pure-pw passwd user
Man page: http://linux.die.net/man/8/pure-pw
I've just done this on Debian-based Raspberry Pi OS with success:
sudo pure-pw passwd {username}
[ENTER NEW PASSWORD WHEN PROMPTED x 2]
sudo pure-pw mkdb
sudo service pure-ftpd restart