Welcome to Android Central! Assuming you're talking about the stock clock/weather widget, long-press an empty part of the homescreen, tap the Add Widget button, then look through your widget list for that particular widget. Long-press it and drag it back to the homescreen.
*Long press an empty space on home screen.
*Tap Widgets icon.
*Weather clock should come up 1st.
*Scroll to left to look at all weather clock widgets
*There should be about 9 to choose from.
*Long press the one you want
*Position where you want on home screen then let go.
*Click 'Done'.
*If you are not satisfied with the one you chose, long press the widget and move to top of screen to red 'remove' letters
Then start steps again.