After enabling the Guest account in your computer, you can rename it by use of the following three ways.
Video guide on how to rename Guest account in Windows 8:
Method 1: Rename Guest account in Control Panel.
Step 1: Log in your PC via the Guest account.
Step 2: Open the Control Panel.
Step 3: Enter User Accounts and Family Safety.
Step 4: Click User Accounts.
Step 5: Tap Change your account name to continue.
Step 6: If you haven't created a password for Administrator account, directly choose Yes in User Account Control dialog to move on. If the Administrator account is password-protected, type the password and tap Yes.
Step 7: In the Change Your Name window, enter a new name and tap Change Name button when done.
Method 2: Change Guest account name in Local Group Policy Editor.
Step 1: Get into Local Group Policy Editor.
Step 2: Find and open the policy called "Accounts: Rename guest account".
FYR, it is located in Computer Configuration/Windows Settings/Security Settings/Local Policies/Security Options.
Step 3: In the following window, input new name and tap OK.
Method 3: Rename Guest account in Computer Management.
Step 1: Access Computer Management.
Step 2: In Local Users and Groups/Users, right-click Guest and choose Rename to rename it.
Tip: You can also rename the account in its properties window, following the two procedures below.
1. Right-tap Guest account and select Properties.
2. As the Guest Properties dialog box turns up, type new name in the box beside Full name, and click OK.
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