Expanding Theremin's range

https://www.youtube.com/watch?v=j_lsG3Fi2ow

"Can I ask you how can I re-tune it? Which parts would it be?"

If it is an Etherwave, the parts would be L5 and L6, the variable and local pitch oscillator coils, and you would tune them with a plastic hex tool so as not to destroy the ferrite and so as not to interfere with things during the tuning.  I would try removing the pitch antenna an replacing it with a 50pF capacitor to ground just to see if it is even possible (if you have the tuning range to accommodate the static capacitance of a large antenna).  You might have to short out one or more of the EQ coils (L1 thru L4) to achieve tuning.  A scope would be really helpful, when the Theremin is connected to the large antenna you would tune L5 for largest voltage swing on the scope with the scope probe clipped to a short piece of wire placed some inches from the antenna, and afterward tune L6 for the desired heterodyned pitch, with a bunch of L5 & L6 minor tweaking afterward extremely likely.