Changing numbering in a Heading style

[TheQueenBea]

I am revising a document that another company wrote for us and then they went out of business so I am stuck with a fairly complicated mess. It has a Heading 7 that controls all the numbering in the TOC and the footer. I need to change the starting number to 13. This is what shows in the detail box when I click on Modify Style:
Indent:
Left: 0.31"
Hanging: 0.25", Keep with next, Level 7, Numbered + Level: 1 + Numbering Style: 1, 2, 3, … + Start at: 12 + Alignment: Left + Aligned at: 0.31" + Indent at: 0.56", Style: Quick Style
Based on: GOM-3
Following style: Normal

All ‘chapter headings’ that show in the TOC are not numbered in the document but have a custom style called GOM-1. The details on that are:
Font: (Default) Arial, 12 pt, Bold, Font color: Custom Color(RGB(31,73,125)), All caps, Left
Line spacing: single, Space


Before: 12 pt
After: 6 pt, Widow/Orphan control, Keep with next, Style: Quick Style
Following style: GOM-3



I have tried numerous ways to change the 12 to a 13 but I'm not having much luck. I have written in before and none of the suggestions worked. Anyone have any ideas? Thanks!

Judi