This has been answered many times before, please use the search feature in future.
- Go into your AdminCP
- Select "vBulletin Options" from the navigation menu
- Select "vBulletin Options" from the new menu that appears after following Step 2
- From the list in the right panel find and double click the "Style & Template Options" title
- Look for the option "Use Legacy (Vertical) Postbit Template?" and set this to "Yes"
- Remember that any modifications you had made previously to the postbit template will need to be redone to the postbit_legacy template. Otherwise you may find that some customisations will not appear (depending on the modification).