Summarize this article:

There is no way to install mods on a device that is not jailbroken. Jailbreaking your iOS device can be tricky and may cause your phone to stop working or void your warranty. Click here for instructions on attempting to jailbreak your iOS device. Currently, the only Herobrine mods available for iOS are downloaded through Cydia. Many require that Winterboard be installed as well. Note: If you find a Herobrine mod online as a .deb file, you can install it using iFile, which is available on Cydia. This still; requires a jailbroken iOS device. There will likely be several to choose from. Find one with good reviews, or look up YouTube videos to see which one you like the best. Different Herobrine mods will have different features. Use the download link on the Cydia page to download and install the mod through the Cydia package manager. Some mods require you to apply a Winterboard theme in order for the mod to work. To do so, launch Winterboard and then tap the Herobrine mod entry until a blue checkmark appears. You will then need to respring or reboot your device. When you launch Minecraft PE, your Herobrine mod will be installed. The summoning method for Herobrine varies depending on the mod (many just risking the regular Zombie enemies to be Herobrine, so you don't actually summon it).

Summary:
In order to install mods, your iOS device MUST be jailbroken. Open Cydia. Search for a Herobrine mod. Install the mod. Apply the Winterboard theme. Start Minecraft PE.