Be sure you make backups of Kamailio and Siremis databases. Also, you should backup the directory where Siremis was installed. In this way, you can restore back if something goes wrong.
Some notes and tips about upgrade of an existing v4.1.0 installation:
Siremis 4.2.0 continues on top of 4.1.0, the upgrade should be straightforward
copy the files (not the folders) from siremis-4.2.0/* to siremis-4.1.0/*
copy siremis-4.2.0/siremis/modules/ser over siremis-4.1.0/siremis/modules/ser
copy siremis-4.2.0/siremis/modules/sipuser over siremis-4.1.0/siremis/modules/sipuser
copy siremis-4.2.0/siremis/modules/menu over siremis-4.1.0/siremis/modules/menu
copy siremis-4.2.0/siremis/images over siremis-4.1.0/siremis/images
copy siremis-4.2.0/siremis/themes over siremis-4.1.0/siremis/themes
with a web browser, go to Siremis administration, under Modules menu, reload ser and sipuser modules
from Siremis administration panel, got to Cache - Manage System Cache and do Clear All