As default, CII chose CAESAR.FRP file. When I'm trying to select another FRP database file (e.g AMRN55-21.FRP) in Configuration, CII is still keeping parameters from CAESAR.FRP file when chosing mat ID20 in piping input.

How can I drive the software to read other FRP database file (not default) when chosing mat ID20 in piping input??

I assume you already started the model with that "CAESAR.FRP" data set.
After updating the configuration file, go back to your input and re-select material 20. The new data will replace the old data.