failed to fetch http://...... Hash Sum mismatch! what to do?

[katME]

i try to update through command terminal and i got this error, plz do help what to do
i am just newbie... any concern plz do help!!!! huhuhuhuhu

W: Failed to fetch http://ftp.cuhk.edu.hk/pub/Linux/ubuntu/...6/Packages  Hash Sum mismatch

W: Failed to fetch http://ftp.cuhk.edu.hk/pub/Linux/ubuntu/...6/Packages  Hash Sum mismatch

E: Some index files failed to download. They have been ignored, or old ones used instead.