For anyone else that built this from source like I did, here's a bash script that will automatically upgrade your instance to 0.21-rc1:

#!/bin/bash -x

cd <FUNKWHALE_DIRECTORY>
export FUNKWHALE_VERSION="0.21-rc1"

sudo -u funkwhale curl -L -o front.zip "https://dev.funkwhale.audio/funkwhale/funkwhale/builds/artifacts/$FUNKWHALE_VERSION/download?job=build_front"
sudo -u funkwhale unzip -o front.zip
sudo -u funkwhale rm front.zip

sudo -u funkwhale curl -L -o "api-$FUNKWHALE_VERSION.zip" "https://dev.funkwhale.audio/funkwhale/funkwhale/-/jobs/artifacts/$FUNKWHALE_VERSION/download?job=build_api"
sudo -u funkwhale unzip "api-$FUNKWHALE_VERSION.zip" -d extracted
sudo -u funkwhale rm -rf api/ && sudo -u funkwhale mv extracted/api .
sudo -u funkwhale rm -rf extracted

# update os dependencies
sudo api/install_os_dependencies.sh install
sudo -u funkwhale -H -E <FUNKWHALE_DIRECTORY>/virtualenv/bin/pip install -r api/requirements.txt

# collect static files
sudo -u funkwhale -H -E <FUNKWHALE_DIRECTORY>/virtualenv/bin/python api/manage.py collectstatic --no-input

# stop the services
sudo systemctl stop funkwhale.target

# apply database migrations
sudo -u funkwhale -H -E <FUNKWHALE_DIRECTORY>/virtualenv/bin/python api/manage.py migrate

# restart the services
sudo systemctl start funkwhale.target

this is my version for build from source (both api and front), which uses git for update check, systemd with user instance.
this script will sync with develop branch.

#!/bin/bash

pushd /home/funkwhale/live > /dev/null
git fetch --all --tags --prune --prune-tags
git checkout develop > /dev/null 2>&1
git status | grep "up to date"
if [[ $? -eq 0 ]]; then exit 0; fi

git reset --hard origin/develop
GIT_CURRENT_COMMIT=$(git rev-parse HEAD)
UPGRADE_VERSION=$(git describe --tags --exact-match || echo "$(git describe --tags $(git rev-list --tags --max-count=1)) (${GIT_CURRENT_COMMIT:0:8})")
source venv/bin/activate
python -m pip install --upgrade pip wheel setuptools
pip install -r api/requirements.txt
pushd front
npx yarn install --no-dev
npx yarn build
popd

systemctl --user stop funkwhale.target
python api/manage.py migrate --noinput
python api/manage.py collectstatic --noinput
systemctl --user start funkwhale.target
popd > /dev/null

systemctl --user status --full --no-pager funkwhale-server.service funkwhale-beat.service funkwhale-worker.service
/home/funkwhale/notice FunkWhale upgraded to v$UPGRADE_VERSION