You don’t want the third step there - that’s for creating a new virtual environment. The rest will work though:
$ sudo su -s /bin/bash homeassistant
$ cd /srv/homeassistant
$ source bin/activate
$ pip3 install --upgrade homeassistant
The second and third can be combined:
$ sudo su -s /bin/bash homeassistant
$ source /srv/homeassistant/bin/activate
$ pip3 install --upgrade homeassistant
Which takes you back to the steps above