I have 3.4.1 installed now. 1&1 has Moodle 3.3.2 as a preset click and install (you only have to enter your config info and it will set it all up for you). To update I just made a backup first, deleted all the Moodle files from the web folder, replaced them with the unzipped 3.4.1, and then replaced the config.php file with my previous one. I had to change the database setting in config.php to 'dbcollation' => 'utf8_unicode_ci' to make it work because I was unable to switch the mySQL format from Antelope to Barrracuda. I'm not sure if I emailed into 1&1 if they would run the scripting for me to do that (as I don't have root access in myPHPAdmin) but my database is pretty small and I don't need emojis! Currently I am only running a few small courses for work related stuff so I don't think I will have any issues, but if I do then I'll have to go from there.
Cheers!