only way is manually from within the course or using an API.

I have never used API, what is that exactly?

Application Programming Interface, basically how you work within an platform like docebo programatically, instead of through a front end interface. We just did a webinar series on the API, and how you can use a cool tool included in Docebo called the API Browser, might want to check out if interested:

If you get comfortable in the API browser, there are some actions you can do with one call, like these kinds of enrollment requests, theres a quick grabs article on this already:

Otherwise, you have to go into the course, use the multi-select boxes and use the bulk action menu in the bottom right to mark complete in batches.