SCCM servers OS upgrade

Currently running SCCM CB 1802 on 1 Primary and 2 Secondary servers and 75 DPs. All running on 2012 R2.

Need to upgrade the Primary and 2 Secondary server OS to 2016.

Which is the best method either Inplace or Side by side?

If inplace , what are the risk and how to mitigate the issues?

Any document for inplace?

If side by side, how to migrate the existing SCCM CB 1802 to another server with new OS 2016.