If you are intimidated by FTP, but your host provides you with the current version of cPanel, its File Manager does an excellent job of the same thing. Just be careful that you upload into the themes directory.
I'll leave my original post unedited below in case anyone else encounters the same problem.
I've just upgraded WordPress at www.helaineolen.com/blog. The site is a few years old.
it had been built on a theme that is no longer available and does not work with the current vers of WP. As it should, it has reverted to one of the default themes.
So I just tried installing the Atahualpa theme.
At the foot of the Themes pages there is an error msg saying "Broken Themes
The following themes are installed but incomplete. Themes must have a stylesheet and a template.
atahualpa Stylesheet is missing.
Warning: include_once(/home/holen01/public_html/blog/wp-content/themes/atahualpa/functions/bfa_theme_options.php) [function.include-once]: failed to open stream: No such file or directory in /home/holen01/public_html/blog/wp-content/themes/atahualpa/functions.php on line 65
Warning: include_once() [function.include]: Failed opening '/home/holen01/public_html/blog/wp-content/themes/atahualpa/functions/bfa_theme_options.php' for inclusion (include_path='.:/usr/lib/php:/usr/local/lib/php') in /home/holen01/public_html/blog/wp-content/themes/atahualpa/functions.php on line 65
Warning: include_once(/home/holen01/public_html/blog/wp-content/themes/atahualpa/functions/bfa_get_options.php) [function.include-once]: failed to open stream: No such file or directory in /home/holen01/public_html/blog/wp-content/themes/atahualpa/functions.php on line 67
Warning: include_once() [function.include]: Failed opening '/home/holen01/public_html/blog/wp-content/themes/atahualpa/functions/bfa_get_options.php' for inclusion (include_path='.:/usr/lib/php:/usr/local/lib/php') in /home/holen01/public_html/blog/wp-content/themes/atahualpa/functions.php on line 67
FWIW, you will notice I have been able to install another one of the popular WP themes. The two defaults also work fine. So the issue appears to be Atahualpa-specific. If any of the trusted regulars wnat access, I'll send admin and FTP codes in a PM.