You can't without recreating the file system. Basically, you need to backup whatever is on that partition, reformat it and restore from backups.
If you did not change the setting when you created the file system before, you need to use the -N option. Make sure that N is capitalized too. I have no idea on how to set that number tho. I have never ran out of inodes but have read about this problem before. This is one of those you don't want to have because it sneaks up on you. Most people watch the space but never check on the inodes.
You may want to check into other file systems too. The extX family uses inodes but reiserfs doesn't and I think there are others that don't use inodes either.
Oh, each time you create a file, the name of that file is on a inode. So each file uses one inode for the name and where the file is located. That's my understanding of it at least.