Replace character at certain location within string

Question

Given a certain string, e.g., s = \"tesX123\", how can I replace a certain character at a certain location?

In this example, the character at position <

Answer 1

Try substr()

substr(s, 4, 4) <- "t"
> s
#[1] "test123"

Answer 2

We can use sub

sub("(.{3}).", "\\1t", s)
#[1] "test123"