Actually, I did explain that, but I will give a little more detail.
You would need to either use the devices file manager app (if it doesn't have one you can download one from Google Play Store) and you can use it to move files back to the phone, prior to removing the SD card. Then, once the old card is out, insert the new one and use the file manager app to move them over to the new card.
If, however, you don't want to have to do this one or two files at a time, you can always connect the phone to a computer via USB cord and move all the files from the SD card either to the phone or to an empty folder on the computer. These can be moved all at once. Once done, disconnect the phone from the computer, change out the SD cards. Then...