@auxclass is correct.
Themes can't be uploaded to blogs hosted on WordPress.com. They can only be uploaded to self-hosted blogs running on WordPress.org software.
On WordPress.com, you can select from a variety of themes to change your blog's appearance. You can read more about customizing the appearance and theme of your blog on WordPress.com here:
We offer over 210 themes, both free themes and premium themes created by leading theme companies. You can view all of our themes and filter them by specific criteria here:
You can also post suggestions for new themes we might add on our public user forum:
Let us know if you have any more questions.