If you want to protect the whole workbook from being modified, you can save the workbook as a read-only file.

1. Open the workbook, and click File > Save As (> Browse).

2. In the Save As dialog box, please click Tools > General Options. See screenshot:

Notes:
(1) By default, the folder containing the current workbook is opened in the Save as dialog box, and the workbook name is added in the File name box automatically. Please do not change them.
(2) If you have never saved the current workbook before, please specify a folder to save it, and type a name for it in the File name box.

3. In the General Options dialog box, please check the Read-only recommended option, and click the OK button. See screenshot:

4. Click the Save button in the Save As dialog box.

5. Now the Confirm Save As dialog box comes out. Please click the Yes button to go ahead.

Note: If you save the workbook for the first time, this dialog box won’t pop out.

When you reopen the workbook later, a warning dialog box will pop out and tell you the workbook is read-only as below screenshot shown:

If you want to protect a worksheet from being modified, you can save this worksheet as read-only by protecting it easily.

1. Enable the specified worksheet you will save as read-only, and click Review > Protect Sheet. See screenshot:

2. In the Protect Sheet dialog box, please type a password in the Password to unprotect sheet box, uncheck any option in the Allow all users of this worksheet to list box, and click the OK button. See screenshot:

3. In the Confirm Password dialog box, please retype the password, and click the OK button. See screenshot:

Now when you or other users want to modify the worksheet, a warning dialog box will pop out as below screenshot shown:

If you need to protect multiple worksheets from being modified, you can save them as read-only in bulk by Kutools for Excel’s Protect Worksheets feature.

1. Click Kutools Plus > Protect Worksheets > Protect Worksheets. See screenshot:
2. In the Protect Worksheet dialog box, please check the worksheets you will protect, and click the Ok button. See screenshot:

3. In the second Protect Worksheet dialog box, please (1) type a password in both Password and Retype password boxes, (2) uncheck any option in the Allow all users of this worksheet to list box, and then (3) click the Ok button. See screenshot:

4. Now a dialog box comes out and tells you how many worksheets have been protected. Please click the OK button to close it. And then close the first Protect Worksheet dialog box.

Now all specified worksheets are protected. Nobody can modify these worksheets until they are unprotected by a correct password.

Lock and protect selected cells in Excel

Protect / lock hidden columns in Excel