How to upload a sketch via Arduino Nano
I presume you have boot loader on your Nano, so you can use your USB to serial, just wire (D0)TX->RX(USB), (D1)RX->TX(USB), GND-GND and supply power to the target Nano. You will need to reboot Nano just before the uploading the code. Uploading is same as usual.

Using other Arduino as ISP needs more wires as well as 2nd Arduino, so i would use above method.