<p>I have a few ASP.Net database front-end websites where MS Access is the back-end.  I am trying to use ASP.Net dynamic data website.  Should I change this database to SQL Server Express or something else to make it easier or should this work from with MS Access.</p>

Pick SQL Express for these reasons:
Scaling
: MS Access will never scale.  Once you get over about 10 concurrent connections you will start to see trouble.  SQL Express will scale.  You can always move SQL Express up to a full-blow SQL Server installation.  While this is somewhat true of Access, some of your SQL statements and data types may not transfer cleanly.
Security
: SQL Server has a much better security model than Access.  You can lock down the schema in your db per user.  You can also better administrate user access (think dev user vs test user vs production user).
Performance
: This is similar to scaling.  If you see a traffic spike to your web site, Access may not handle it while SQL Server Express probably will.
Tools
: Tools and libraries like LINQ are always going to be targeted at SQL Server.  You will get better support and better documentation using them this way.