How can I write a script in Access Database (use as front end) to re-link all the tables with the tables in SQL server automatically when I open the "startup" form in Access?.

If you search the web, you can find the code pretty easy. It is VBA code.

You don't need to re-link the tables at startup although, if the tables change frequently or may be moved around, you should verify the links and give the users a graceful way to "find" the new database.

Code samples can be found in the Microsoft knowledge base and in solutions.mdb. Try this one:

ACC Procedure to Create Data Sources and Relink ODBC Tables (Q159691)

The solutions example db has code to relink or to verify links if I recall
Dave