The correct answer to this open question is the following.
Although there are no options attached or specific references, we can say the following.
The Upanishads reading describe those who will be "born no more (reach union with Brahman)" in that the 13 Upanishads establish that humans have to strictly observe the rituals to find their place in the vast universe. Doing this, humans could understand the not always easy to understand ideas of the Supreme Creator that Hindus call Brahman. This is the concept of God for other religions.
The reunion with Brahman would be through Atman, which is the higher aspect of humans or higher self in the Upanishads. The goal is to reunite once more with Brahman through the divine spark or soul, for eternity.
Those who will not have the conditions to reunite with the Creative Force of the Universe will continue to live in the illusion of duality in the Earth dimensions.
