Which device? The procedures are different for some. If it connect to your pc/mac through a usb cable, I'd go to the page in Garmin Connect where is displays the summary list of your activities and select the 'import' function then browse to the folder on your device where the activity is stored and select the appropriate file. On my edge500, that would be /garmin/activities.
If you are using a forerunner that uses an ant stick, I think you have to go into it's setup menus on your device and tell it to upload all. But I don't have a forerunner, so I'm not too sure of how and where to do that on a forerunner.
