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docs/How_To_Update_ONNX_Dev_Notes.md

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+This is a note only for ONNX Runtime developers.
+
+It's very often, you need to update the ONNX submodule to a newer version in the upstream. Please follow the steps below, don't miss any!
+
+1. Update the ONNX subfolder        
+```
+cd cmake/external/onnx
+git remote update
+git reset --hard <commit_id>
+cd ..
+git add onnx
+```
+(Change the <commit_id> to yours. If you are not sure, use 'origin/master'. Like 'git reset --hard origin/master')
+
+2. Update [cgmanifest.json](/cgmanifest.json)        
+Search 'https://github.com/onnx/onnx.git', update the commitHash with it.
+
+3. Update [tools/ci_build/github/linux/docker/scripts/install_deps.sh](/tools/ci_build/github/linux/docker/scripts/install_deps.sh)        
+Search 'for onnx_version in', update the commit hashes. The list should contain every release version from ONNX 1.2, and the latest one in our cmake/external/onnx folder.
+
+4. Update onnxruntime/core/protobuf        
+If there is any change on `cmake/external/onnx/onnx/*.in.proto` since the last sync, please apply these changes to `onnxruntime/core/protobuf/*.in.proto`
+
+Then copy cmake/external/onnx/onnx/gen_proto.py to onnxruntime/core/protobuf, use that script generating the new \*.proto and \*.proto3 files.
+
+Regenerate csharp/test/Microsoft.ML.OnnxRuntime.Tests/OnnxMl.cs
+
+5. Send you PR, and run the CI builds.        
+
+6. If there is any unitest failure, caught by onnx_test_runner. Please also update        
+- [onnxruntime/test/onnx/main.cc](/onnxruntime/test/onnx/main.cc)
+- [onnxruntime/test/python/onnx_backend_test_series.py](/onnxruntime/test/python/onnx_backend_test_series.py)
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