overwrite existing metafields

I use metfields Pro to import spreadsheets of metafields for all of my products. The values keys of my metafields change often but the key stays the same so I want to make sure that if I upload a sheet of metafields with the key "tof" and the value is "false" and the next time if I upload a spreadsheet with the same key and the value is "true", that it will OVERWRITE/update the existing metafields and not just create two metafields with "tof":"false" and "tof":true as that would defeat the purpose.