How does one change riding lawn mower blades?

Most of the time just jack the front end, put a blocks behind the back tires and loosen the bolt and drop them down. Some models you have to hold the nut on the topside while you turn the bolt.

Are these bolts usually standard or reverse thread?

Check the brief sticky here:

https://www.askmehelpdesk.com/tools-power-equipment/frequently-asked-questions-answered-our-experts-235047.html