First of all, do you have an Yahoo India id or Yahoo international site id?
If you have Yahoo India id, You can enable POP3 from Options and download all the mails to Outlook or outlook express or Windows Live Mail Desktop beta and store them locally. Then you can delete them from your inbox.
Otherwise, check wether you have saved your sent mails and delete the unnecesary ones.
Also, you can forward the mails to some other id and delete them from your yahoo mailbox.