If I am not mistaken, there are two time zone settings for phpBB - there is the global time zone setting for the board which can be set via the Administration Panel's General Admin - Configuration section where you can select the time zone via the System Timezone setting under the General Board Settings section. This is a dropdown and should be fairly straightforward.

The other timezone setting is under the user profile and is again a drop down under the Preferences section named Timezone. So since both are dropdowns, the changing of the time zone should be fairly easy. Unless I'm missing something? If so, an indication as to which setting you are trying to change would help