Replace patterns separated by delimiter in R

sub(".*(?<=_)(\d+)$", "MAP_\1", tt, perl = T)
[1] "MAP_62770" "MAP_6272"  "MAP_1222"

Here we use positive lookbehind to assert that there is an underscore _ on the left of the capturing group (\d+) at the very end of the string ($); we recall that capturing group with \1 in the replacement argument to sub and move MAP_in front of it.

An option with trimws from base R along with paste. We specify the whitespace as characters (.*) till the _. Thus, it removes the substring till the last _ and then with paste concatenate a new string ("MAP_")

paste0("MAP_", trimws(tt, whitespace = ".*_"))
#[1] "MAP_62770" "MAP_6272"  "MAP_1222" 

You can use:

gsub("^CBII_.*_.*_", "MAP_",tt)

or

stringr::str_replace(tt, "^CBII_.*_.*_", "MAP_")

Output

[1] "MAP_62770" "MAP_6272"  "MAP_1222"