An electron of mass m and magnitude of charge $\left|e\right|$ initially at rest gets accelerated by a constant electric field E. The rate of changes of de-Brogile wavelength of  this electron at time t ignoring relativistic effects is 

Debroglie wavelength of  changed particle moving with velocity $\upsilon is{\lambda }_D=\frac{h}{mv}$ given that an electron of mass m and magnitude of  charges $\left|e\right|$  initially at rest gets accelerated by a constant electric field E. Acceleration of  particle is ,
    $a=\frac{eE}{m}$
 As charged particle is  travelling with  constant  acceleration and started from rest, velocity of particle at time  t is   $\upsilon =at$
$\upsilon =\frac{eE}{m}t$
Debrogile wavelength of  charged particle moving with velocity   $\upsilon =is{\lambda }_D=\frac{h}{m\upsilon }$

$$\begin{gathered} \Rightarrow {\lambda }_D=\frac{h}{m\left(\frac{eE}{m}\right)t} \\ \Rightarrow {\lambda }_D=\frac{h}{eEt} \end{gathered}$$
Rate of  change of  Debrogile wavelength of  charged particle with time  t is   $\frac{d{\lambda }_d}{dt}=\frac{d}{dt}\left(\frac{h}{eEt}\right)$

$\Rightarrow \frac{d{\lambda }_d}{dt}=-\frac{h}{\left|e\right|E{t}^2}$