The direction of change in velocity ($\delta v$) is given by
$\phi=\frac{180^{\circ}-\theta}{2}=90^{\circ}-\frac{\theta}{2} \,\,\,\,\,\,\, ...(i)$
This can be shown graphically as
For small time interval, i.e., $\delta t \rightarrow 0$, then angle between $v_{1}$ and $v_{2}$ is very small i.e., $\theta \approx 0^{\circ}$. So, from Eq. (i), we get
$\phi=90^{\circ}$