Hi, I have a quick question. Is there a way to update the summary checkbox via CSV import?

I don’t think so, but you can check by looking for the Summary field in the CSV Import field list for Account CSV Import. Just start an import with the internal ids of the accounts you want to modify, and look for the field on the right-side. If it’s not there, it’s not supported by CSV Import. In the past I’ve written scripts to load/edit/save in bulk, when the CSV Import isn’t viable for updates.

It's annoying after 5 years, it's still not there. What I've done in the past is add a custom account field called "Summary-temp" or something. That will show up in the CSV importer. Import to that field as required. Then put together a simple workflow that copies the checkbox into the standard system Summary field. Then delete the custom field. It's clunky but it works.

@RJMNS there are many things which will never be improved.. this is one of them.