Yes, you can change your modules in an evergreen course even if it has already started.
To add modules to your course, follow the instructions here: How do I add modules to a Course?
Your modules will be released to your group depending on your module release settings.
Once you have updated the modules or rearranged them, they will be updated on the Modules page of your clients' dashboard.
Relevant Article:
How can I re-arrange or re-organize my modules inside a course?