Note: only the User with the persona/rights is able to do this task. The Persona required is [Buyer - Administrator]. The personas can be found in this article, User permissions - New Global Personas.
Please follow the below steps.
1. From 'Dashboard', go to [System] [Access] [Users].
2. Click on the 'blue' 'eye' icon of the User's name to update.
3. Click on the [Details] tab.
4. Click the 'yellow' [Change Password] button.
5. In the pop-up window, type in the new password, and type the password in a second time to verify the change, click [Save].