jmc

We subtract the rightmost digits as usual. However, for the next digits, we need to borrow as shown: $$\begin{array}{cccccc}& & & \text{cancelto}01& \text{cancelto}92& 6_7\\ & -& & & 5& 4_7\\ \text{cline}2-6& & & & 4& 2_7\end{array}$$ Thus, the answer is $\boxed{42_7}.$