diff --git a/docs/ios/build_instructions.md b/docs/ios/build_instructions.md
index 188fd0bd..b381794 100644
--- a/docs/ios/build_instructions.md
+++ b/docs/ios/build_instructions.md
@@ -13,7 +13,7 @@
 ## System requirements
 
 * A 64-bit Mac running 10.12.6 or later.
-* [Xcode](https://developer.apple.com/xcode) 11.4+.
+* [Xcode](https://developer.apple.com/xcode) 12.0 or higher.
 * The current version of the JDK (required for the Closure compiler).
 
 ## Install `depot_tools`
@@ -328,6 +328,42 @@
 [waterfall](https://build.chromium.org/buildbot/waterfall/) is green and the tree
 is open before checking out. This will increase your chances of success.
 
+### Changing the version of Xcode
+
+To change the version of Xcode used to build Chromium on iOS, please follow
+the steps below:
+
+1.  Launch the new version of Xcode.app.
+
+    This is required as Xcode may need to install some components into
+    the system before the new version can be used from the command-line.
+
+1.  Reboot your computer.
+
+    This is required as some of Xcode components are daemons that are not
+    automatically stopped when updating Xcode, and command-line tools will
+    fail if the daemon version is incompatible (usually `actool` fails).
+
+1.  Run `gn gen`.
+
+    This is required as the `ninja` files generated by `gn` encodes some
+    information about Xcode (notably the path to the SDK, ...) that will
+    change with each version. It is not possible to have `ninja` re-run
+    `gn gen` automatically when those changes unfortunately.
+
+    If you have a downstream chekout, run `gclient runhooks` instead of
+    `gn gen` as it will ensure that `gn gen` will be run automatically
+    for all possible combination of target and configuration from within
+    Xcode.
+
+If you skip some of those steps, the build may occasionally succeed, but
+it has been observed in the past that those steps are required in the
+vast majority of the situation. Please save yourself some painful build
+debugging and follow them.
+
+If you use `xcode-select` to switch between multiple version of Xcode,
+you will have to follow the same steps.
+
 ### Improving performance of `git status`
 
 #### Increase the vnode cache size