This is the opposite to my last question case.

Let $F$ be a totally real number field, $R$ is a quaternion algebra over $F$ unramified in at least one infinite place of $F$. Let $\mathcal{O}⊂R$ be an order.

Can I explicitly compute the rank and generators of the group of units $\mathcal{O}^{\times}$ in terms of, say, some basis in $R/F$?

It seems that the rank is related to the corresponding Shimura curve, but I do not see how this helps.