Steps
Screenshots are from GNOME but most GUI desktops are similar.
Select install tarball wherever you have it downloads (often in your
Downloads
directory), right-click and select ‘Extract To…’File manager showing extraction of install tarball
Browse to the boot partition of the SD card
Initial directory when choosing directory in which to extract install tarball
Select the boot partition of the SD card as extraction destination
We have selected the boot partition as the destination for extracting the install tarball
The result will be a directory containing the boot files on the boot partition; we actually want the files in the top-level (root) of the boot partition.
File manager showing directory creating boot files created in the boot partition
Therefore, select all files in the directory that was created (e.g. in the file manager, browse into the directory and press [Ctrl-A]).
File manager showing all files boot files in directory from tarball selected
Cut and paste the files into the top-level (root).
File manager showing the boot files moved into the top level of the boot partition
Remove the now extraneous extraction directory (which should be empty).
File manager showing result of removing the now extraneous extraction directory