That's a big job
Where do you start?
The ISS is in an inclined orbit, geostationary requires one aligned with the equator. Changing inclination is a big job,
About half of the energy used to lift a kilo to geostationary orbit is used getting it into low orbit, the remainder is used in the transfer to the higher orbit. So take whatever energy its been to assemble the ISS and double it.
You'd then have to contend with the fact the ISS would lie outside the inner Van Allen Belt and would be exposed to more radiation and necessitate its crews being exposed to a healthy dose of whatever the Sun coughed up a few days ago.
And finally, we don't have any manned vessels that can get there.