Is it possible to turn a 12 volt battery to 6 volt battery?

Answer 1

You cannot make a 6 volt battery into a 12 volt batter, but you can make a 12 volt battery out of two 6 volt batteries. Connect two 6 volts batteries together in series and you have 12 volts. Please define "in series".

Answer 2

Depends on the cell design.

Something like car batteries will be very hard to convert from 12 to 6 volts, as the interconnects between the cells are part of the carrying structure of the battery as a whole.

But something like a Ni-Dc battery will probably be possible to turn from 12 to 6 volts, as that battery is more likely to consist of structurally independent cells in a common housing. The cells can be dismounted from each other and reused in another configuration.