Go to Brackets (Manage > Brackets/Round-Robin > Brackets)
For one Bracket:
1. Select the bracket
2. Pencil to edit,
3. Advanced section,
Change the Ext Match Id Start Number.
For multiple brackets that should follow Match numbers as though they were one big bracket:
Select all the brackets that go together. *Tip: Hold Ctrl and select all the ones you need.
Click on Display More Options (Top Right).
Click on Change Number Offset.
Put in Ext Match Id.