You can set the sync rate in Tools > Options > Syncronisation, the quickest rate is every 5 mins, or you can manually sync by hitting the Syncronise button in the bottom left (I think Ctrl+S works too).
(Note: the app doesn't do it in the background, you have to hit Syncronise manually)
Also worth noting that if you get conflicts from two versions syncing before the other one gets there, Joplin will preserve the conflicting notes and let you pick what to keep from each
