How to update the C++ sources of gRPC:

1. git clone http://github.com/grpc/grpc.git in a convenient directory
2. git checkout <tag> (current is v1.18.0, commithash 007b721f)
3. mkdir -p third_party/grpc/src
4. cp -R <gRPC git tree>/src/{compiler,core,cpp} third_party/grpc/src
5. cp -R <gRPC git tree>/include third_party/grpc
6. rm -rf third_party/grpc/src/core/tsi/test_creds
7. Update BUILD files by copying the rules from the BUILD file of gRPC; fix macros in third_party/grpc/build_defs.bzl if necessary
8. Apply local patches if necessary: patch -p3 < netinet_tcp_h.patch
9. Update //third_party/nanopb if necessary

How to update the Java plugin:

1. Checkout tag v1.10.0 from https://github.com/grpc/grpc-java
2. cp -R <grpc-java git tree>/compiler/src/java_plugin third_party/grpc/compiler/src

How to update the Java code:

Download the necessary jars at version 1.20.0 from maven central.