Hi Klaus,
Please start with Overview document below and then proceed to read the others listed on left under Migrate to Azure Resource Manager. I recommend performing the migration during off hours since there will be some downtime while VM(s) are stopped/deallocated/started.
Platform-supported migration of IaaS resources from classic to Azure Resource Manager
https://learn.microsoft.com/en-us/azure/virtual-machines/migration-classic-resource-manager-overview
Thanks.
-TP