**Readability**

 - Use namespace `CompanyX` instead of using in every method's return type.
 - Create single instance of `InvoiceTableAdapter` and `SqlConnection` and reuse it.
 - In `UpdateInvoice` method, instead of passing multiple properties, pass data model itself and use properties which are required inside `Update1` method.
 - You will need to put the cleanup of the connection in a `Dispose()` after making the data access class `IDisposable`. As pointed out by [ratchet freak][1] in another answer.

Not sure why you're using 2 SQL connections. I would suggest you to use only one.

After refactoring code should look like this:

        using CompanyX;
        using CompanyX.DataAccess.CompanyXTableAdapters;
    
        namespace CompanyX.DataAccess
        {
            public class CompanyXDataAccess : IDisposable
            {
                InvoiceTableAdapter ta = new InvoiceTableAdapter();
                SqlConnection readerConnection = new SqlConnection(DB.GetReaderConnectionString());
                SqlConnection writerConnection = new SqlConnection(DB.GetWriterConnectionString());
            
                public InvoiceDataTable GetInvoiceByID(int ID)
                {
                    ta.Connection = readerConnection;
                    return ta.GetByID(ID);
                }
    
                public InvoiceDataTable GetInvoice(string userSID)
                {
                    ta.Connection = readerConnection;
                    return ta.Get(userSID);
                }
    
                public void DeleteInvoice(int ID)
                {
                    ta.Connection = writerConnection;
                    ta.Delete1(ID);
                }
    
                public void UpdateInvoice(Invoice m)
                {
                    ta.Connection = writerConnection                                    
                    ta.Update1(m);
                }

                public void Dispose()
                {
                    // Do clean up
                }
            }
        }


  [1]: https://codereview.stackexchange.com/users/32061/ratchet-freak