Thanks Dason! You're absolutely right it was way easier in base R. Thanks for the perspective.
df <- data.frame(x1=c(1,9,10), x2=c(11,43,3))
df[df==1|df==2|df==3|df==4|df==5] <- NA
1 NA 11
2 9 43
3 10 NA
Any insight to as why the following doesn't work?