Can I use the import wizard tool in Toad to update rows to a table without truncating the table?
Unfortunately, the Import Wizard is strictly used to Insert new rows of data to an existing table or new table.
With that said, a solution that will work is to use the Import Wizard and import all data to a new table which can be created using the wizard on the fly. Instead of choosing your existing table, create a table with same name in another schema.
Once it's created with the new data in the new table, perform a Data Compare. In the Data Compare wizard, select the new table as the source, then the target table you intend to update. In the Compare results see if it detects the changes desired. If it does, then perform a Synchronization to make the Target like the Source. Then choose the option to load the script to the Editor and will generate a script of Update SQL statements to update the table.