ftp error when automatic update

when i try u automatic upgrade plugin or try to update 2.7 to 2.7.1 in next window
1. when i give ftp user name & currunt password message recive password not correct, when i give my old ftp password process start
. down loading compleat unpaking compleat etc & when i click on reactivate plugin in new window msg recived file not exist.
how can wordpress upload with my old ftp password.
how can i resolve this problem? how can i update plugin automaticaly?
plz help me.