In Young's double slit experiment the separation $d$ between the slits is 2 mm, the wavelength of the light used is 5896 \AA and distance $D$ between the screen and slits is 100 cm. It is found that the angular width of the fringes is $0.20^\circ$. To increase the fringe angular width to $0.21^\circ$ (with same $\lambda$ and $D$) the separation between the slits needs to be changed to