[sectionA]
url = value1
username = value2
password = value3

[sectionC]
url = value1
username = value2
password = value3

[sectionB]
url = value1
username = value2
password = value3

sed '/sectionB/,/[/s/username.*/username = valueX/' input

awk -vRS='' -vFS='n' -vOFS='n' '
$1~/sectionB/{
    sub(/=.*$/, "= valueX", $3)
}
{
    printf "%snn", $0
}' input

sed -E -n '1h;1!H;${;g;s/[sectionB]([^[]*)username = [a-zA-Z0-9_]+/[sectionB]1username = valueX/g;p;}' input.txt