This guide will tell you how to get Overextended in Warframe.
Overextended is only dropped from opening Orokin vaults on Deimos, which you can find in any mission. Before anything, you need to make sure that you bring all 4 dragon keys so you can open the vault each time you run the mission.
It is recommended to bring a frame that is hard to kill or one that you can survive easily with. In this case, it’s Rhino but you can use whoever you’d like since carrying all the dragon keys will make it very easy for you to die in the mission.
Overextended does have a low drop chance but it should get you a mod each run. It might take you a few or a lot of runs before you actually get the mod you’re looking for. You are going to be looking for a door with a large symbol on it in the mission.