If the id is always unique and does not duplicate, then try:
Read the updated version of excel file with manual changes as a datatable 1, then read the ERP import as a datatable 2. Outer join them in to a new datatable 3 then append dt3 the original dt1. I have seen some folks posting linq queries to do similar to get the new rows here:
prashant1603765
gorby
sharazkm32
pabaleloh