Can 4G phone be upgraded to 5G by software update?

No, this is not possible to update via software in the future .11 May 2021

A software update can quickly turn the current 4G LTE networks into 5G . “This is a game changer,” said Qualcomm’s Malladi.

Most of the 5G smartphones in India come with support for N78 bands. This band operates within 3200 MHz to 4200 MHz frequency range.