For rotating the display by 90 degrees change
dtoverlay=tft35a
to
dtoverlay=tft35a:rotate=90
and reboot.
Note that touch has to be rotated, too. If touch currently is correctly aligned to the display then run
sudo curl -L https://raw.githubusercontent.com/goodtft/LCD-show/master/usr/99-calibration.conf-35-180 -o /etc/X11/xorg.conf.d/99-calibration.conf
to follow a rotation of the display by 90 degrees.