proftpd configuration problem

Hi ,i'm using proftp daemon on my server.

The problem is that i access with a user with anonymoys directive .
for example

<Anonymous /data/www/html>

User ftp
Group inet
Umask 002
AnonRequirePassword on
UserAlias myuser ftp
AuthUsingAlias on

RequireValidShell off
AllowOverwrite on

ShowSymlinks on

</Anonymous>

the problem is that i can read ,write ,delete files ,but i can't update
files (same as delete one file and after that upload same file updated).

What may i do ?

Thanks .