Sure. Here's how to do it. I am assuming you are on a Linux-like computer, or have RTools installed on Windows. Here I am being charitable and including MacOS as a Linux like computer, but you may need XCode or some other stuff that I don't care to know about if you are on that OS.

$ git clone https://git.bioconductor.org:/packages/Biostrings
Cloning into 'Biostrings'...
remote: Enumerating objects: 17997, done.
remote: Counting objects: 100% (17997/17997), done.
remote: Compressing objects: 100% (4633/4633), done.
remote: Total 17997 (delta 14546), reused 16366 (delta 13322), pack-reused 0
Receiving objects: 100% (17997/17997), 31.09 MiB | 26.12 MiB/s, done.
Resolving deltas: 100% (14546/14546), done.

## cd into the directory and checkout the release version

$ cd Biostrings/
$ git checkout RELEASE_3_14
Branch RELEASE_3_14 set up to track remote branch RELEASE_3_14 from origin.
Switched to a new branch 'RELEASE_3_14'
## Go into the R directory and change the TAG
## you could just open the GENETIC_CODE.R file in an editor and change as well,
## but is that what the cool kids do? I didn't think so
$ cd R/
$ sed -i 's/TAG="\*"/TAG="X"/' GENETIC_CODE.R
## now back out and start R
$ cd ../../
$ R

R version 4.1.2 (2021-11-01) -- "Bird Hippie"
Copyright (C) 2021 The R Foundation for Statistical Computing
Platform: x86_64-pc-linux-gnu (64-bit)

<snip>
## install the package. If you are on Windows, you need an additional argument of type = "source"
> install.packages("Biostrings", repos = NULL)
Installing package into ‘/share/groups/edge/R/R-4.1.2’

<snip>

* DONE (Biostrings)
## load the library
> library(Biostrings)
Loading required package: BiocGenerics
<snip>

## check the GENETIC_CODE
> Biostrings:::GENETIC_CODE
TTT TTC TTA TTG TCT TCC TCA TCG TAT TAC TAA TAG TGT TGC TGA TGG CTT CTC CTA CTG
"F" "F" "L" "L" "S" "S" "S" "S" "Y" "Y" "*" "X" "C" "C" "*" "W" "L" "L" "L" "L"
CCT CCC CCA CCG CAT CAC CAA CAG CGT CGC CGA CGG ATT ATC ATA ATG ACT ACC ACA ACG
"P" "P" "P" "P" "H" "H" "Q" "Q" "R" "R" "R" "R" "I" "I" "I" "M" "T" "T" "T" "T"
AAT AAC AAA AAG AGT AGC AGA AGG GTT GTC GTA GTG GCT GCC GCA GCG GAT GAC GAA GAG
"N" "N" "K" "K" "S" "S" "R" "R" "V" "V" "V" "V" "A" "A" "A" "A" "D" "D" "E" "E"
GGT GGC GGA GGG
"G" "G" "G" "G"
attr(,"alt_init_codons")
[1] "TTG" "CTG"