Simply by the changing of the DNA.
Meiosis is the process where a cell with 2n chromosomes divide to produce 4 new cells with n number of chromosomes each with different genotypes which is how single-celled organisms reproduce and reproductive cells divide. This creates different genetic combinations therefore supporting diversity and natural selection.
Crossing Over is a procedure that is a part of the meiosis. It is when the chromosomes are replicated and at a total of 4n, the homologous pairs of the chromosomes have a cross-over of DNA, resulting in genetic diversity.
Mutations, even though they are harmful most of the time, are also a process where genetic diversion accours because of either mistakes in the cell divisons or external effects such as radiation. Therefore mutations are also contributing to the allele diversity.
I hope this answer helps.