go to the specific vendor with the corrections, drill into the ledger entries. Make sure you show 1099 code and 1099 amount columns after drilldown. highlight the transaction you need to update, select the 1099 code box and press f2. this should let you edit the 1099 code, you can do the same for the amount.
the 1099 reports refer to these ledger entries to populate the 1099 reports.