I started out with learning Katakana characters because that's what I mostly come across. Now I started Words for Japanese Basics, and the words are Kanji with Hiragana. There doesn't seem to be a way to change it to Katakana. Is there, or should I go back and learn Hiragana now?
So, there is not really a way to change between the two - most words are written in hiragana (and kanji if you know them), and some are written in katakana - it is not a decision that the writer can make, and it usually deals with the origin of the word.
The J Basics lessons do have katakana words in them, but they are sparse in comparison to hiragana/kanji ones (which is the case with Japanese in general).
If you'd like to focus on some katakana only lessons, you can search for Katakana in the Lesson Center (under Resources in the menu). There are a lot of user-made lessons available there that are collections of katakana terms.
