Pure-FTPd on Debian uses a configuration file hierarchy below /etc/pure-ftpd/conf.

For example, to set the upload and download bandwith to 2Mbit (=250 KB) per second,

create /etc/pure-ftpd/conf/UserBandwidth with the following values:

250:250

Restart Pure-FTPd. Please note that existing connections aren't affected by that change.