There doesn't seems to be any graph API available for access/update file present on specific Microsoft Teams channel.
We can get the metadata for the location where the files of a channel are stored using following API
GET /teams/{id}/channels/{id}/filesFolder
Ref Doc : https://learn.microsoft.com/en-us/graph/api/channel-get-filesfolder?view=graph-rest-1.0&tabs=http
Thanks,
Nivedipa
If the response is helpful, please click "Accept Answer" and upvote it. You can share your feedback via Microsoft Teams Developer Feedback link.