Proftpd user/password : Easy question

Hi

Well it's an easy question but I could find an answer and I'm a
beginner using unix...
I'm working on a FC1 server with proftpd installed

Several site has been configured and each one has a different
user/password to access the site by ftp.

Although i did find the list of users allowed for each site, I don't
know how to update the password :

For example, if user1 has pwd1 as password for site1, how can i change
his ftp access to user1 / pwd2 for site1?

Thanks for your help !