Hi-

Okay, you can check out this thread for explicit instructions on how to update your R to 3.5.3 on Ubuntu 18.04 here:
[afni.nimh.nih.gov]

Well, I guess I can repost the instructions here so they are in a single post:

Remove your current R:

sudo apt-get remove r-base-core

Add the following line to the bottom of your /etc/apt/sources.list file *but also remove the underscores "_" as you do*:

deb h_t_t_p_s_:_/_/cloud.r-project.org/bin/linux/ubuntu bionic-cran35

(I had to add the underscores for the forum post, just so it wouldn't be tranlated to an abbreviated link). To open that file so that it is editable, you need to use sudo when calling your text editor; so, something like: "sudo gedit /etc/apt/sources.list" to open the file, then copy+paste that line, then remove the underscores; save+close that text file.

After that, run:

sudo apt-get update

so that the package manager can see the changes.

Then run:

sudo apt-get install -y r-base-dev

to install R, which should be some v3.5.* now.

Then, run this to try to reinstall necessary packages.

rPkgsInstall -pkgs ALL