Migration guide to v10?

[wereHamster]

I have an app which uses getInstallationAccessToken(), and that function is not available anymore in the new version v10. I would appreciate a migration guide which explains how to migrate my code.

[gr2m]

you can find the migration guide in the README of the previous version:
https://github.com/octokit/app.js/tree/v4.3.0#%EF%B8%8F-deprecated

You want to migrate to @octokit/auth-app, not v10 of @octokit/app. Before v10, @octokit/app was just a means of receiving authentication tokens. It now is a fully fledged API to send requests, iterate through all installations/repositories, handle webhook requests and oauth flows.