Both are fine, but the expression is usually "broaden [or deepen] one's [or his, her etc] knowledge of [something]".

Hello, Both are correct. Deepen your knowledge of ....... (something). Broaden your knowledge of .....(something). Regards

Depends on what you want to say. Broadening your knowledge is understood to mean that you would like to cover the full span of knowledge of a subject. Deepening your knowledge refers to the extent to which specific topics are focused upon, amplified and explored. In other words, you are deepening your understanding of a specific area of interest as opposed to broadening the scope of your knowledge of an area of interest