It sometimes happens that you might find it difficult that the wordpress automatic upgrade plugin is not working well enough as it halts in between the process When it tries to unpack the .zip file and then it stops.Although  in previous WordPress versions there was a plugin titled WP Automatic Upgrade that, as you might guess, automatically upgraded blogs. That functionality is now built-in to WordPress from 2.7 onwards.

I have faced this problem once and have got its answers

So if you have wp automatic plugin working then you should disable it first if your blog version is above 2.7

For older versions :
There might be the problem of path setting
yes when you choose to upgrade it asks for root of the blog and the username password.Please make sure that you are using the right combinations of them…