mass change characteristic value

Hi,

I've Created Characteristic - size - with charcateristic values 10 , 20 & Assigned to CLASS: CLASS TYPE 001, this i've assigned in material master - in classification tab & have set SIZE= 10.

i want to change  characteristic value from 10 to 20, In 5000 materials numbers

i'm unable to see classification tab details in MASS tcode
can you Pls. suggest how i can mass change characteristic value in material master.