This is a function only available using the Corrections module and records can be merged using the following steps:
In Dimensions, on the Creditors tab, click Suppliers.
Select the supplier to merge with another. This supplier record will disappear.
From the Corrections or Plug-Ins menu, click Correct .
From the new Corrections window that opens, click Actions, Merge.
Enter the code of the Supplier to merge with. This record will remain.
Click the Green Tick icon and confirm.