Can you untap an attacking creature to use its ability at the end of declare blockers?
Yes you can.

The combat phase consists of five steps:


beginning of combat
declare attackers
declare blockers
combat damage
end of combat


In each of these steps players get priority and can cast instants or activate abilities. In the declare blockers step the first thing you do is declaring the blockers. It is after this that each player gets priority and can do stuff. And this is before the combat damage step