If there's no `passwd` or `chpasswd` try this jank

password_change_manually.sh

#!/bin/sh

set -eu

# DEBUG=true
DEBUG=false

if $DEBUG; then
    trap 'echo "# $BASH_COMMAND";read' DEBUG
fi

PAGER=${PAGER:-more}
for cmd in sed openssl diff $PAGER; do
    if ! command -v "$cmd" >/dev/null 2>&1; then
        echo "Missing required command: $cmd" >&2
        exit 1
    fi
done

if [ "$(id -u)" -ne 0 ]; then
    echo "You must run this as root." >&2
    exit 1
fi

if ! cat /etc/shadow > /dev/null; then
    echo "Cannot read /etc/shadow" >&2
    exit 1
fi

printf "What user are we messing with? " >&2
read -r target_user

shadow_line=$(grep "^$target_user:" /etc/shadow || true)
if [ -z "$shadow_line" ]; then
    echo "User '$target_user' not found in /etc/shadow" >&2
    exit 1
fi

IFS='$' read -r _ algo salt current_hash <<EOF
$(echo "$shadow_line" | cut -d: -f2)
EOF

if ! [ -n "$algo" ] && [ -n "$salt" ] && [ -n "$current_hash" ]; then
    echo "Failed to extract hash components." >&2
    echo "shadow_line:  ${shadow_line:-"unset"}"
    echo "algo:         ${algo:-"unset"}"
    echo "salt:         ${salt:-"unset"}"
    echo "current_hash: ${current_hash:-"unset"}"
    exit 1
fi

echo "Sanity check: if you know the current password, enter it now."
echo "Or press Enter to skip:"
read -r current_pw
if [ -n "$current_pw" ]; then
    computed_hash=$(openssl passwd -$algo -salt "$salt" "$current_pw")
    expected_output="\$$algo\$$salt\$$current_hash"
    if [ "$computed_hash" != "$expected_output" ]; then
        echo "Password does not match!"      >&2
        echo "Computed from your input:"     >&2
        echo "$computed_hash"                >&2
        echo "What appears in /etc/shadow:"  >&2
        echo "$expected_output"              >&2
        echo "The full line in /etc/shadow:" >&2
        echo "$shadow_line"                  >&2
        exit 1
    else
        echo "Password verified."
    fi
else
    echo "You have FOOLISHLY chosen to skip password verification." >&2
fi

printf "Enter new password: " >&2
read -r new_pw
# openssl will output the dollar signs, algorithm, salt, and hash.
new_full_hash=$(openssl passwd -$algo -salt "$salt" "$new_pw")

# Assigning to a variable seems to turn newlines into spaces, I'm sure it could be fixed, but this works fine.
cat /etc/shadow | sed "s#^$target_user:\$$algo\$$salt\$$current_hash:#$target_user:$new_full_hash:#" > /etc/shadow.new && \
echo "Wrote /etc/shadow.new" >&2

echo "Here is a diff of the changes:" >&2
echo "---" >&2
diff /etc/shadow /etc/shadow.new | $PAGER
echo "---" >&2


printf "Type 'lgtm' to apply changes (a backup will be made): " >&2
read -r confirm
if [ "$confirm" != "lgtm" ]; then
    echo "Deleting /etc/shadow.new and aborting." >&2
    rm /etc/shadow.new
    exit 1
fi

echo "Applying changes..." >&2
cp /etc/shadow /etc/shadow.bak
cat /etc/shadow.new > /etc/shadow
rm /etc/shadow.new
cat <<EOF

 !!!!!!!!!!!!!!!!!!!!!!!!!!! 
 /!\ /etc/shadow updated /!\ 
 !!!!!!!!!!!!!!!!!!!!!!!!!!!

Test login now. If it fails, run:
  mv /etc/shadow.bak /etc/shadow

If everything looks good, run:
  rm /etc/shadow.bak

EOF