If you have changed your user account from an Administrator account to a standard user or guest, you have basically locked yourself out of making any meaningful changes in Windows and you may need to reset or reinstall Windows to resolve this problem.
1
Try this method to make your user account an administrator account first.
Click the power button on the Start Menu
Press the Shift key and click Restart.
Your PC will boot into the Windows Recovery Environment.
Go to Troubleshoot - Advanced Options - Startup Settings and click Restart.
Upon restart, press 4 to enter Safe Mode.
Type Administrator as the user on the login screen and leave the password box empty when logging into Windows as an Administrator.
In Windows, click the Start button, then type netplwiz and press Enter.
In the resulting utility, select your user account and click Properties.
Select the 'Group membership' tab and set your account as a member of the administrators group.
Save the settings and restart the PC.
2
If that process is not successful, backup your files to ensure you do not loose any data, then form inside the recovery environment, choose the reset option and select the option to keep nothing.