I want to change an existing subscription but my only option is to view my subscription or cancel it
If there isn't any option to change it then you may only have one option if you want to subscribe through Roku. Generally, subscribing directly with the provider gives you more options and allows you to use the subscription on devices other than Roku.
I want to change my ESPN subscription TO monthly and no longer yearly
Thanks for the post.
For more information about that channel's subscription plans, you'll want to contact ESPN support directly to inquire further. Many channels on Roku are developed and maintained by the channel provider themselves.
You can reach ESPN support here: https://support.espn.com/hc/en-us
Thanks,
Danny