@FraPochetti
I wouldn’t recommend upgrading to 1.3.1 if you intend to use SageMaker to train with Estimators. The notebook instance is currently running 1.3.0 and the training and inference instances only support 1.3.0, 1.2.1, 1.1.0, 1.0.0, 0.12.1. The training instances run a version of MXNet dictated by the framework_version kwarg in Estimator.
If you were just using SageMaker’s notebook as a hosted notebook instance you can update the version of MXNet to 1.3.1 as follows but you will not be able to train/infer with this version.