Dunno, try deleting nupgrade.sh1 in case it already exists. If it already exists, who owns it? It would appear you've done something outside of the instructions - maybe you've tried to download the file multiple times which is why wget is trying to create additional versions; maybe you did this first as root which is why "user" can't create the file or maybe the entire directory is not writeable by "user"... who knows? Follow the instructions - they work. Invent your own, you're on your own (not really, we'll try and help!)

The linked article is providing step-by-step instructions - did you follow them? They don't get much easier than that, to be honest. As they are known to work, I suspect you've done something wrong - my money would be on the directory you have created being owned by root and thus not writeable by user. Check the directory ownership, and either change ownership to user (chown users:user ~/bin) or delete the directory as root (rm -fr ~/bin) and recreate when logged in as user.