Can I turn on a 12V relay with 5V supply, just by changing a resistor?
Here's how to run a 12V relay from 5V.

Remove the relay from the PCB.
Disassemble the relay, removing the coil from the rest of the mechanism.
Unwind the relay, counting the turns as you do so.
Divide the number of turns by 2.4, measure the wire thickness, and buy enamelled wire with approximately twice the cross sectional area (rounding up if possible).
Rewind the coil with the new wire, and the reduced turns count calculated at step 4
Reassemble the relay and re-fit to the PCB.

In practice, just buy the matching 5V relay suggested in the duplicate question.