What's the key of life when the Sun turns into a red giant and the Earth burns up and explodes?

The key of WHAT life? There's probably other life in the universe to which the demise of the earth will be of no consequence. If you are referring to human life and you are also referring to the survival of the species, the "key" is not philosophical, it's technological.  Can mankind migrate to another habitable solar system? Not likely given present circumstances.  

Ages to ashes dust to dust. The human created in an imploded planet in a fiery blaze where death comes in seconds, leaves in deductive reasoning the ashes of his existence. Whether a spirit liberated. A soul released, or the earthly elements of earthly attachments, airy affectations, watery subsistence charged negatively or positively, or fiery ardour or fiery tempestuousness, we are cells of a collective, and in so being should ensure we are free from the parasitic attribute of earthly existence and survival, though these are requisite should we bot have reached the heights of heavenly being or sensual living, thus is the tragedy of life and living, detachments from one is essential for becoming the other one. Of liberating, releasing becoming free. That is truth. This is existence. There is life to be lived should we be knowledgable to distance from the parasitic life cycle of eating sleeping working, to the sensory condition of being requiring the elimination of sin, or vice, or hate or the toxins that create hazardous waste and antisocial behaviour in the human. Figure them their trespasses and move on, as you are forgiven your trespasses. Move on.

In 5.4 billion years from now, the Sun will enter what is known as the Red Giant phase of its evolution. This will begin once all hydrogen is exhausted in the core and the inert helium ash that has built up there becomes unstable and collapses under its own weight.

The key to life is knowing Jesus Christ as your personal Lord and Savior. 

If the Earth and the Sun perish, the key to human life is in God's hand, as it always has been. It never has been in human hands or minds, since all humans die.

"Dossier on the Ascension" by Serapis Bey.

The Bible assures us: “Generations come and generations go, but the earth remains forever.” (Ecclesiastes 1:4,) God securely “founded the earth upon its established places” and designed it to last “to time indefinite, or forever.” (Psalm 104:5)

There is a “fire” that warms our skin by day. That “fire” is burning some 93 million miles (150 million km) away! What power the sun must have for you to be able to feel its heat from such a distance! Yet, the earth orbits that awesome thermonuclear furnace at just the right distance. Too close, and earth’s water would vaporize; too far, and it would all freeze. Either extreme would render our planet lifeless. Essential to life on earth, sunlight is also clean and efficient, not to mention delightful.—Ecclesiastes 11:7.

The key to life is found at Revelation 14:7-Fear God and give him glory.

The earth will soon dissolve like snow,

The sun forebear to shine;

But God who called me here below,

Will be forever mine. 

it only burns if you are too close so I guess the key would be not to be here when it happens that is of course IF humanity is not already dead...

N.Shadows

Relocation, it seems