Best way to add and update a custom to client's MS teams app.

RishavKatoch -

  1.There are two ways to publish a custom app and the way that you use depends on how you get the app. To publish a custom app by uploading an app package (in .zip format)that a developer sends you. The other method, approving a custom app, is used when a developer submits an app directly to the Manage apps page through the Teams App Submission API.
When you publish a custom Teams app, it's available to users in your organization's app store.

2.The access token contains information about your app and the permissions it has to access the resources and APIs available through Microsoft Graph. To get an access token, your app must be registered with the Microsoft identity platform and be authorized by either a user or an administrator to access the Microsoft Graph resources it needs.

Graph API- 

POST  https://login.microsoftonline.com/{tenant}/oauth2/v2.0/token

Host: login.microsoftonline.com
Content-Type: application/x-www-form-urlencoded

client_id=535fb089-9ff3-47b6-9bfb-4f1264799865 &scope=https%3A%2F%2Fgraph.microsoft.com%2F.default &client_secret=sampleCredentia1s &grant_type=client_credentials