Hi Trish,
While you can't make this type of change on a single label - you can change the font for the form as a whole, which in turn would change the font of the labels. Under Form Settings in the General area you can select a "Font Family" that you want to apply to the form. This article can illustrate this further:
https://community.marketo.com/MarketoArticle?id=kA050000000LHAPCA4