If you've used Google Docs before, this will feel familiar. Before you change ownership, make sure you invite the new owner to your Coda doc as an editor.
Once you have them in the doc, go to Google Drive and right click on the document and select Share. Then click Advanced in the bottom right corner. Lastly, click the dropdown arrow next to the person who you'd like to be the new owner, and change his or her role to owner. Don't forget to save the changes!