The yellow on blue is unreadable.
For processing xml files you might want to look at the xsltproc program.
sed '/bst0a/,/bst0e/s/brTim=".*"/brTim="something_else"/' testfile
If the bst0<letter> patterns are in order and brTim= is contained within the range, then you can use sed easily.
The slashes before the s command select a range of lines based on a regex pattern. You can also use 1,/bst0e/ as well to start the range on the first line.