Try this:
sed “s/aaa=.*/aaa=xxx/g”
You can also use sed’s change line to accomplish this:
sed -i “/aaa=/caaa=xxx” your_file_here
This will go through and find any lines that pass the aaa= test, which means that the line contains the letters aaa=. Then it replaces the entire line with aaa=xxx. You can add a ^ at the beginning of the test to make sure you only get the lines that start with aaa= but that’s up to you.