From phpBB Development Wiki
Using Migrations in your Extension is very simple to do.
Automatically install/revert Migrations when enabled/purged
If you wish to have your Migrations installed and removed automatically when your extension is enabled and purged respectively, all you need to do is put your Migration files in ext/(your extension name)/migrations/
If the user has updated your extension and new migration files were added, you can call the following function:
Any new migration files included in the migrations/ directory in your extension will then be automatically installed.