Preparations before upgrading
backup the forum
backup the database of the forum
remove non invision themes
remove non invision apps
remove non invision hooks
check upload paths in admin panel
You could put the board in dev mode and run everything highlighted in the red warning boxes disabled when done.
check in phpmyadmin the database collation it must be set on utf8mb4_unicode_ci
if you have done all of this let's prepare the new files
remove all directories but not the uploads, public or the files one if you have downloads enabled
remove the rest of the root index but keep conf_global.php
upload our latest release as @Setup mentioned above
extract it where the 3.4.9 files were
go to yourdomain.com/admin/convertutf8/ to prepare your database for the upgrade
now you are ready to install the upgrade by just going to yourdomain.com
goodluck