What did you try?
It is quite easy to change using ifelse:
First we create some fake data with the dots
a <- c(3,4,5,'.',6,7,6,'.',4,5,'.')
a
[1] "3" "4" "5" "." "6" "7" "6" "." "4" "5" "."
We change dots with NA using ifelse, and make the output numeric:
a <- as.numeric(ifelse(a =='.', NA, a))
a
[1] 3 4 5 NA 6 7 6 NA 4 5 NA
But we don't know how your data looks and the exact error/problem you are facing 