If you have a recording headset, you may try changing the electrode location if possible. If you just have the data, it does not make sense to find it. Just use what you have.

Thanks Amr Elsawy

I have a headset (Emotiv epoc) and it does not record data from center locations. also, it has fixed channel locations. locations can't be changed.

how can be this problem solved.

one more point,

If data from nearby locations are available, e.g. C3 and C4.

Can average of these be used as Cz?

Ashish Aggarwal for Emotiv headset some people wear it in reverse to have other locations, but they surely lose the original locations.

You didn't record data from Cz, so there's no way to recover that (just like there's no way to recover a photo you never took). The best you can do is approximate/estimate what Cz might look like. Some people do this by creating "virtual channels"; you interpolate between the channels that you do have (using the same methods that are used to make a topographic plot) and then take the interpolated data from where Cz would be. So it's essentially a weighted average of data from channels close to where Cz would be. There aren't many situations, however, where you would need that. If you want to compare your data to previous results, it would be better to just look at the channels that you actually did record and that are near Cz.

Thanks Amr Elsawy