There are a couple of ways:

1. Using the Manage Accounts control:

From the start screen type "account", switch to the "Settings" filter on the right, and then select "Change Account Type".

Select the user you want to change. and then "Change the account type", Select the "Administrator" type, and then press "Change Account Type".

This is probably the preferred way, as it's very obvious which account you are changing.

2. Using the Computer Management console:

Either Windows+x and select "Computer Management".

Open the "Local Users and Groups" part of the "System Tools", and then "Users".

Double click on the user you wish to make a member of the Administrators Group and then open the "Member Of" tab - Press "Add..." and then type "administrators" into the text area. Press "Check Names", and then OK.

If your accounts have the same display names, this method may be a little confusing as the user accounts will all be based on the first name with some additional characters.

Thanks @Karan for your help. I contacted Microsoft Support and thay had me run the hardware troubleshooter, which fixed the problem.