Switch to Edit Mode in Excel
When opening Excel spreadsheets from unknown sources, a feature in Excel called Protected View shields you from potential risks. This is great, but to work on the document, you will need to switch to editing mode.
How to Switch to Editing Mode
Protected View is on by default. Whenever you open a file that originated from a website or that you received via email, you are taken to Protected View.
The message bar will appear along the top of the document and offer you the chance to enable editing. Click the “Enable Editing” button.

The document opens as read-only with some content disabled.

Click on File > Save As in order to Save the document to another location to be able to work on it effectively.
Why Use the Protected View?
Protected View protects you from malware, viruses, and threats that can be exposed to when opening Excel files from unknown sources. These sources include a website, email attachments, and other unsafe locations such as shared drives.
If you are just viewing the spreadsheet, you will not need editing mode and can happily work with the document while in Protected View.