Append and replacing data in r
You could change in this case with command 

game_results$winner[1] &lt;- "Dad"


But this copies the whole table, so i will recomend you to create an empty data in another way 

game_results &lt;- setNames(data.table(matrix(nrow = 10, ncol = 3)), c("winner","round","winnerscore"))


and assignment by reference in the way you prefer (hole column or specific cell), here instructions for both cases.