Hi @Greg68 ,
Could you please tell me:
Do you mean the second list has the same Title column as the first list?
Is the lookup column of your first list set based on the title column of the second list?
I have made a test for your reference:


1. You can use two get items to get the items of the two lists respectively.
2. Add two apply to each to traverse the value of the two lists respectively.

3. Add a condition under apply to each to determine whether the titles of the two lists are the same.
4. If the same, use update item, fill in the ID of the second list in the lookup id.

Best Regards,
Levi