We have a lot of genome-editing tools – like zinc finger nucleases, or CRISPR/Cas9 systems – that could theoretically allow you to epigenetically seek out and turn on genes that make your muscles physically large, make you strategically minded, incredibly fast, or increase your stamina. Of course, at this point, these types of things have been explored mostly in lab mice, but it's fun to speculate.
Changing animals by putting human genes or cells into their structure is one way of making them more resemble the bit of the human condition you're interested in studying.
The entire “library” of genetic instructions that an organism inherits is called its genome. A typical human cell has two similar sets of chromosomes, and each set has approximately 3 billion nucleotide pairs of DNA. If the one-letter abbreviations for the nucleotides of a set were written in letters the size of those you are now reading, the genetic text would fill about 700 biology textbooks.