More specifically, after you download, you will see CHIRP's "spreadsheet" populated with your current "memorys". You can now modify that spreadsheet to change the existing memories or fill in "memories" that are currently "empty". You may need to hit the "show empty memories" button to cause the "empty" lines to show up so you can fill one of them in. CHIRP cannot cause your radio to suddenly have additional memory capacity, so if your radio has only 20 memories, CHIRP will display that many only.
After you mess around with the "spreadsheet" display, you can then "upload to radio" to cause the to be put back into the radio.
Hint: After you download and before you make changes, save the image. After you change the image and before you upload, save to another file.