Under Administration -> Backups and Upgrades -> Settings, you will have previously set a domain account on this screen, and this is the account that needs to have permissions to write to that file. It does a bunch of other stuff too, like deleting old files and copying in new ones, and the easiest way to fix this is to make it a member of the local administrators group on your web server.
If you start the upgrade again, hopefully it goes through this time, if everything is ok. If you run into any issues, click the Test Permissions button on that Backup and Upgrades page and this will give you more information. Password may be incorrect, or locked out etc.
Hope this helps,