how to convert .mdf file to .mdb or .accdb

I suppose you are talking about migrating a database from SQL Server to Access.
Well the sort answer is: forget it, because SQL Server database is far more complex than Access database, thus you can loose very-very much of it's content. But if you want to transfer only the data, that's possible. In SQL Manager there is an import-export feature. Have you tried it?
If you want to do the same in c#, follow these steps:
1) Open source (sql) database
2) Open or create destination (access) database
3) For all tables in source:
3.1) Create the same structure in destination (without reference constraints)
3.2) Copy data from source to destination
3.3) Create reference constraints in destination
4) For all views in source:
4.1) Create views in destination (might have to add some logic, since the functions you can reference are not the same - this is why data export in SQL Manager is creating tables from views)
But, if you want to have a lightweight, embedded kind of sql server to be able to deploy it with your application, than you better use SQL Server compact edition[^]