How to increment by hand a sequence number.

I've exported a table from an existing MSAccess database to my
PostgreSQL db.
I use a serial ID on the table and now I'm getting errors when I add a
new entry in the table if i don't specify the ID by hand (which should
be placed automatically by the db since it's a serial).

The exact error is this one:

Cannot insert a duplicate key into a unique index.

I think I should set the "last_value" field in the sequence to my real
last value aon the ID field of the table, but Idon't know how to do it.
Is there anybody who can help me ?

Thanks

Valerio Santinelli
tanis(at)mediacom(dot)it