How do I export my wallet transactions?

Galleon does not yet let you export your transactions. However, Cryptonomic's Arronax tool does provide such an option. You may fund instructions for using Arronax for account exports at https://medium.com/the-cryptonomic-aperiodical/using-arronax-to-export-your-tezos-baking-rewards-just-in-time-for-tax-season-ed7f65abf9ac.