Hi Jose,
Yes, we can update existing Leads records by uploading Excel.
It is divided into three steps: ? export the data to an Excel file and download it, ? modify the data in the Excel file, ? import the modified Excel file into D365.
For example, I want to modify the "Business Phone" field of Leads.
Here I share a quick way.
Step 1
Open the Advanced Find, search for Leads, select a view, you can add new filters on the view, the purpose is to search out all records to be modified.

Click Edit Columns to add the fields you want to modify.

Click Result to view the search results. We can see that the fields to be modified have been added. Click Export Leads > Static Worksheet to download it.

Step 2
Open the downloaded Excel file and modify records. All field values must conform to the field format.
Step 3
Go to a Lead view, click Import form Excel. Upload the modified file.
After finishing import, go to Advanced Settings > Data Management > Imports to see the import result.