I assume this is on an iPad or iPhone. There are two ways. You can either move them across one at a time directly on the device, or move them on mass using iTunes with the device connected to a desktop machine.
To do so directly on the device: open a file in Smart Office 2 and use the share feature to "Open in Smart Office Plus".
To do so with iTunes: connect the device to your PC or Mac, open iTunes, click on the devices icon in the left pane and then on "Apps" in the top menu. Within that window, scroll down to "File Sharing". You can save all your Smart Office 2 documents to the desktop machine and then add them back to Smart Office Plus