Ron, In my opinion, your question has nothing to do with servlets, it has to do with JDBC.
There are JDBC drivers that allow you to read data from Excel spreadsheets. Just do an Internet search for the terms "excel" and "JDBC".
Alternatively, you may be able to use the POI project to read the Excel data.
Once you have read the data (from Excel), you can use JDBC (again) to insert the data into your PostgreSQL database. [Of-course, you will need to obtain the relevant JDBC driver for PostgreSQL. This is not the same driver as required for Excel.]