Completing a PRL update will provide your phone with the most up-to-date information related to roaming and the roaming network.
When taken along with a Profile update, a PRL update is a part of the standard stream of updates your phone will go through during its initial activation and synchronization with the network.
Because of the incredible variation amongst devices, there is some variation in where you'll locate the PRL command in your device's menu settings.
Common paths on Android phones include:
Settings > System Updates > Update PRL
Settings > About Phone > Update PRL
A common non-smartphone path to a PRL Update:
Menu > Tools > Update Phone > Update PRL
Windows phones and iPhones do not have a separate PRL update command.
If you have any other questions about activating over the network, and PRL & profile updates, you can find more info here.