How does crossing over increase genetic diversity?

Genetic diversity is the fact that the organisms have differents genes or versions of it (allele).

You can be homozygot and get twice the same allele (example : O blood type) or being heterozygot and get two different alleles (ex : AB blood type).

Crossing-over happens during the meiosis when the pair chromosomes cross-over each other and some genetic exchanges may happen, so it can create new allelic combination in the gamete and then, if it leads to a new individual after fecondation, development etc...., get an individual to have new allelic combination and maybe give him advantages, disadvantages or none of them !

But there will be a new allelic combination, and so genetic diversity got increased.

Hope it helps you ! :)