There are plenty of articles covering this, but most of them walk you through installing features, etc. Here’s a quick braindump of one of my migrations of DHCP from a Windows Server 2003 DHCP server to a new one running Server 2012 R2.

On the Server 2012 R2 server:

1. Elevated PowerShell prompt, run: Install-WindowsFeature Migration
2. Run the migration tools command prompt from Server Manager->Tools
3. Create a folder to store your migration package: C:tempWIN2K3MIG
4. Create a package for the downlevel server

C:Windowssystem32ServerMigrationTools> .SmigDeploy.exe /package /architecture x86 /os WS03 /path C:tempWIN2K3MIG

On the Server 2003 server:

1. Copy the package created in step 4 above to the source server
2. Stop the DHCP server service (PowerShell: Stop-Service DHCPServer)
3. Run SmigDeploy.exe out of the package folder, a PowerShell prompt will appear
4. PowerShell: Get-SmigServerFeature
5. Create a folder to hold your exported config: md C:tempsmig_dhcp
6. PowerShell: Export-SmigServerSetting -FeatureId DHCP -Path C:tempsmig_dhcp

Back on the Server 2012 R2 server again:

1. Elevated PowerShell prompt, run: Add-PSSnapin Microsoft.Windows.ServerManager.Migration
2. PowerShell: Stop-Service DHCPServer
3. PowerShell: Import-SmigServerSetting -FeatureId DHCP -Path ‘\2003serverc$tempsmig_dhcp‘ -Force -Verbose
4. PowerShell: Start-Service DHCPServer
5. Authorise the new DHCP server, and de-authorise the old one