To truncate table contents before loading data into it just follow the steps below:

1. Go to workflow manager
2. Create new session on mapping created before in designer
3. Right click session and hit edit…
4. Go to mapping tab
5. Select a target from left column
6. Under properties section check “truncate target table option”

Caution: This will truncate whole data present in target table!

Using: Informatica Power Center v 9.5.0