In order to do so, move to the Users (1) list and click the relevant button (2) under the Operations column.
As soon as you click on this button you are transferred to the user's profile (3) and you can switch back to your admin account at any time, using the green button on the top menu (4).
You can also login into a user's account from within his profile info page. Just click on the more button (4) and select the option "Log into account" (5).
Please note that any action that the admin performs while being logged in as the other user, is reported, including the time that the admin spends in a course or the system in total. You have to be careful while using this feature.