Last year the Astros had a .682 Defensive Efficiency Rating (DER). That is the percentage of balls in play that are turned into outs. The league average was .689.
This year the Astros lead the AL with .721 while the league average is .692.
To get a rough idea of how much difference this might make, I ran a regression on all major league teams from 2014-2017 with ERA as the dependent variable and DER and FIP (fielding independent ERA) as the explanatory variables. Here is the equation
ERA = 11.97 - 17.22*DER + 0.973*FIP
The Astros are .039 better this year at DER (.721 - .682). That times 17.22 gives us .67. So their better defense is estimated to lower their ERA by .67 compared to last year.
Their FIP ERA is 3.16 this year while last year it was 3.91. That is an improvement of 0.75. That times .973 gives us about .73. So the FIP ERA improvement has lowered the overall ERA by more than the DER has, but it is very close.