Run in powershell on local azure ad connect server (FCIUTIL4)
Start-ADSyncSyncCycle -PolicyType Delta (NOTE: This will only sync current changes)
Start-ADSyncSyncCycle -PolicyType Initial (NOTE: This will force a complete sync but note that the length of sync time would be greatly increased.)
PowerShell Basics: How to Force AzureAD Connect to Sync (microsoft.com)