Does DC electricity have a frequency and how to change it?

DC is unidirectional current there is no meaning of frequency because no cycle . dc signal magnitude is same at all stages of time ... in pulsating dc frequency is present..

DC contains pulsated frequency. for ex: AC50 hz applied to an half wave Rectifier pulsated frequency is50 hz. AC50 hz applied to an full wave Rectifier pulsated frequency is100 hz.

No frequency for direct current, we can change DC to AC by using converter device

Frequency (pulse) is a measure of iteration cyclic event, for example frequency sound wave or frequency of light wave or electromagnetic wave. And measure the frequency and severity unit hertz (Hz), which is equivalent (1/sec) In the DC current is fixed at every moment and thus be the frequency of the wave through the second one of the infinite number Applying the previous law frequency in the DC non-existent

But you could give it a frequent by switching it in a certain tempo. This will give a block wave, other than a sinus wave, but still it has a frequency although being DC switched in and off and on again and so on and on.

A lot of people consider it to have a frequency of zero. That works out mathematically for most stuff.When I say most stuff, that's pretty much everything I could think of.For inductive impedance (2*pi*f*L) or capacitative impedance1/(2*pi*f*C) and the imdedance goes to zero and infinity as f goes to zero which is in line with empiracal observations.Most of what I think of is in signal analysis - fourier transforms and FFTs.In these bin zero is DC and all other bins are AC components. For finite signals DC always ends up solely in bin zero or zero frequency. Pulses and steps are combinations of DC and lots of AC components, and sine/square waves symmetrically around zero are AC only. Pulse trains have DC and AC components plus a strong component at the rep rate of the pulse.Of course that give rise to the old philosophical question is there really a DC because it implies it is that way forever, practical sources like a power supply have to be switched on so represents a step. For finite signal analysis (like FFT) the time limit is only the anlysis period.

Yes and we change it (Cycle) with a controled rectifier , we talk about average value in this case  (DC component ). 

It is used to control DC apliance in general (even DC motor too)

no Dc have no frequency but can be converted to Alternating current by invertor (widely available in the market .

pulsating is possible by switching0/1   but no sinosadle is possible  by direct means .  

DC stands for Direct Current it does not have frequency. It can either be classified in ON and OFF States.