In Davission and Germer experiment, if applied accelerating voltage is increased. Then, diffracted electron beam will have the maximum at a value of $\mathrm{\theta }$ that 

The wavelength of the wave associated with an electron accelerated through a given potential difference of V is:

$\mathrm{\lambda } = \frac{12.24}{\sqrt{\mathrm{V}}}\mathrm{\AA }$

According to Bragg's law, the relation between wavelength and angle of scattering is given by:

$$\begin{gathered} \mathrm{\lambda } = \mathrm{D} \mathrm{sin\varphi } \\ \Rightarrow \mathrm{D} \mathrm{sin\varphi } = \frac{12.24}{\sqrt{\mathrm{V}}} \end{gathered}$$

From the above relation, if applied voltage is increased, then diffracted beam will have maximum at the value of $\mathrm{\theta }$ that will be less than the earlier value.