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This is in the top left corner of the screen. Take care not to forget this password; if you lose it, you won't be able to open your file. If prompted, click Save Changes first. You should see a field that reads "(YourFile).xlsx is protected". If you typed in your password correctly, you should now have access to your document!
Open your Excel document. Click File. Click Protect Workbook. Click "Encrypt with Password". Type in a password. Click OK. Re-enter your password. Click OK. Close your document. Re-open your document. Type in your password. Click OK.