We can upsert(Insert/Update) records using External Id field in Salesforce.

1. Use the following link to setup Connected App to get Access Token.

Salesforce OAuth 2.0 Client Credentials Flow to get Access Token

2. We can do a PATCH Request to do the Upsert operation.

Endpoint URL:

https://{Salesforce My Domain UR}/services/data/v59.0/sobjects/{Object API Name}/{External Id Field API Name}/{External Id Field Value}/

Request Header:

Request Body:

{
    "Description" : "Sample Testing"
}