Theme update, mapping and dns

Hi! Newbie here and would really like some help. I’m really confused as how I should proceed right now. My wordpress site is myheartworks.wordpress.com. Not sure of the version, cant seem to find where it is?

My main problem:
I want to install new wordpress themes from themeforest to my current site but can’t find the install themes button?

1. I think its because of my hosting/mapping. So I added a domain (www.heartworks.my) to my unlimited hosting at hostgator. Then I went to update the DNS – but it doesnt work and it cant be done. Is this because I have a domain mapped for my WP site via http://www.heartworks.my? Will doing this enable me to upload a new theme?

2. Would it be wise for me to cancel the mapping and just install wordpress via hostgator using Fantastico into my original URL (heartworks.my) and delete the wordpress site (myheartworks.wordpress.com) and just redo it? I’m happy to do that as I need to give it a big face lift.

Any suggestions would be great. Thank you very much!