Hello,
here is the best way to handle this situation:
1. when you have to update existing record => select the record/records -> action(export) -> check "i want to update..." -> select the required fields -> export.

- this will allow you to export th External Id of this record "unique per every record on the database"

- the excel file will has the first row with technical name of fields which have been chosen in step 1, which will automatically matches the existing fields during import process

2. update/add records

3. activate developer mode -> go to import -> choose your update file-> test-> import

Note: make sure to check "i want to update..." during export process and make sure to import based on this external ID to update this specific record

for simplicity those are main fields to import

hope this will help

Mapping field names during data import into Odoo can be a real sticking point, especially if you've run into a mismatch between exported field names and the mapping options during import. I get why you're frustrated! So, here's the deal: when you export data from Odoo, it sometimes uses technical field names which might not line up with the user-friendly names shown during import mapping. This mismatch can really trip folks up.

First, make sure when you exported the invoice, you picked the option to include field names. It's handy for cross-referencing to map fields correctly during re-import. If the field names you need aren't showing up, it might be becuase those fields aren't active in your modle or linked to another modle tied to the invoice. Just because you see a field in your UI doesn’t mean it’s good to go for import/export.

How do you tackle this? You'll want to head over to Settings > Technical > Database Structure > Fields in Odoo to check out the exact technical names of the fields. Compare with what's exported. Often, with a bit of digging under "Fields," you'll find a clue as to why they’re being elusive. And if a custom module changed these fields or added new ones, it can make 'em even trickier to spot.

If you're familiar with XML for imports, it's another route worth exploring. It gives more control over field mapping, though it's a bit more involved than using CSVs. Really, it's just about knowledge - knowing what each field represents and how Odoo interprets those during import. And hey, if you're stuck, hit up the Odoo community forums or documentation - they’re pure gold for this.