How does the speaker seek to rejuvenate himself through art in "Sailing to Byzantium" by William Butler Yeats?

The speaker in this poem by W.B. Yeats is, we may infer from the opening line, an old man, concerned that, in his old age, he has become "but a paltry thing." He feels that to be old is to be little more than a "tattered coat upon a stick, unless / Soul clap its hands and sing"—that is, the soul of a person must be stimulated in order to keep that person spiritually alive. It is in pursuit of this goal that the speaker has embarked upon a journey to Byzantium, that "holy city."
Rejuvenation, as such, is not the speaker's goal. He does not wish to be young again, but for the "sages standing in God's holy fire" to gather him "into the artifice of eternity." He speaks of leaving his body and becoming, instead of something shaped like a "natural" creation, something "of hammered gold and gold enamelling," which could then exist as it has always existed, singing "of what is past, or passing, or to come." Through the art of the holy city, then, the speaker seeks to somehow transmutate from a living human being into something immortal in its "artifice," filled with song.
The idea of songs and singing is an interesting concept in this poem. Once he is "out of nature," the speaker still imagines himself "set upon a golden bough to sing," as if a knowledge of everything in the universe, in the past, present and future, might become part of his song, although he is no longer "natural." His soul needs to sing, or else he will be "but a paltry thing." And yet, in the opening stanza of the poem, the birds in the trees, too, are "at their song," and yet they are described as "these dying generations." The poem seems to suggest that, through their singing, even the birds seek to keep themselves from death.

In "Sailing to Byzantium," Yeats sees the creation of art as a possible method of escaping one's natural old age and mortality. The poem begins with a description of a world of life in conflict with the certainty of death. The speaker in the poem muses on the fragility of old age, and much of the poem focuses on ways of reversing, or at least remedying, this fragility. Some of the key lines pertaining to this theme occur in the final stanza. For example, the speaker says he wants to be reborn in "such a form as Grecian goldsmiths make / Of hammered gold and gold enamelling / To keep a drowsy Emperor awake" (27-9). In these lines, the speaker is asserting that artistic creation can sidestep old age and death, and that the artistic creative process has the ability to rejuvenate old age by making one essentially immortal. All in all, Yeats presents art as something timeless, something that can sing "Of what is past, or passing, or to come" (32), and so only by creating and engaging with art can one rejuvenate one's old age and escape one's inevitable mortality. 
https://www.poetryfoundation.org/poems/43291/sailing-to-byzantium