residual graph and augmenting path in max flow
A full edge, e.g. $a \rightarrow c$, has a residual capacity of $0$ in the residual network. So you can't make an augmenting path over that directed edge.
However the reversed edge, $c \rightarrow a$ has a residual capacity of $5$ (since $c_{c \rightarrow a} = 0$ and $f_{c \rightarrow a} = -5$). Therefore you can create an augmenting path using the reversed edge.
The residual network would be:



E.g. a possible augmenting path would be $s \rightarrow c \rightarrow a \rightarrow d \rightarrow t$. You can increase the flow along that path by 5 and get the following flow network:



This is also the maximal flow, since $\{s, c\} - \{a, b, d, t\}$ forms a saturated s-t-cut.