Sometimes Agents die during the course of a mission, or you've simply stopped having fun playing them. The following section covers replacing Agents.
If an Agent is killed during the course of a mission, you can choose to make your replacement at the same Tier as the Agent who died, following all other restrictions laid out on this website and in the Core Rules. You can either;
use an Archetype designed for your current Tier
use a lower-Tier Archetype and take an Ascension Package to bring them up to the correct power level, as described in Creation Ascension (Core Rules pg. 148)
Regardless of your choice, your new Agent starts at Rank 1 and with XP equal to 100 x your Tier
If you simply wish to retire an Agent (either to try a new perspective or because they've become unsatisfying to play), speak to a GM. Once they have approved the retirement, you can make your replacement using the same process as described above under Killed In Action.