[SalesForce] Data Import Wizard not updating field

When performing an upsert you can avoid duplicates by matching on either the Salesforce ID or custom External ID type field. If your performing an upsert, or update using dataloader.io, you must map a column to the Salesforce ID or an external ID.

Using dataloader.io, the ~ steps: Import your parent Account records.

1. Select the Import option
2. Select the import operation type – upsert.
3. Select the object you wish to import data into Account.
4. Verify Mapping Between Fields
5. Click Next Upload the CSV file you wish to import by clicking the Upload CSV button.

Import your child Contact records, ensure the Account field contain the name of your Accounts.

1. Select the Import option
2. Select the import operation type – upsert.
3. Select the object you wish to import data into Contact.
4. Verify Mapping Between Fields
5. You'll need to Lookup the Account Ids you've just imported, on the mapping file, select your Account field, and choose the "Lookup via Account" Account Name.
6. Click Next Upload the CSV file you wish to import by clicking the Upload CSV button.

Note, because you are upserting from an export ensure your upsert files do not contain any system fields.

Good Luck.

My advice is to recheck below noted points properly in your org and verify it.

The Data Import Wizard respects Salesforce admins’ field access as specified in page layouts and field-level security. This feature is available in both Lightning Experience and Salesforce Classic.

As per the Page Layouts and Field-Level Security documentation, page layouts determine the fields that Users can import data into. If Users are reporting fields are no longer available for mapping when using the Data Import Wizard an administrator will need to ensure that the affected Users now have appropriate access set for the missing fields. Resolution
Ensure that the affected users profile's assigned record types and their associated page layouts include the missing fields.

If the fields are contained on the page layouts, check field level security for the missing fields on the importing user's assigned profile. You can view field access by field, profile or record type via Setup, type "Field" in the quick find box and click Field Accessibility under the Security Controls menu.

To import leads via the Data Import Wizard: Import Leads permission for user needed and “Read,” “Create,” “Edit,” and “Delete” on the objects

If you can’t import field values that you’ve successfully imported in the past, check these settings. In your assigned profiles, check that your page layout assignments and field-level security settings allow you to see and edit the fields. In your assigned permission sets, check that the field permissions allow you to read and edit the fields. User permissions, such as “Edit Read Only Fields,” also affect whether you can import field values, but that behavior hasn’t changed in this release.

Reference:-

• https://help.salesforce.com/articleView?id=000232755&type=1
• https://releasenotes.docs.salesforce.com/en-us/spring16/release-notes/rn_forcecom_data_diw_field_access.htm
• https://help.salesforce.com/articleView?id=faq_import_general_permissions.htm&type=5

Hope it helps you.