Use

sed '/#start/,/#end/replace_command'

For example, if the file is called myconfig, and you want to replace "allow" with "deny" in that section, you could say

sed '/#start/,/#end/s/allow/deny/' myconfig

example

sed -i -r "/<elasticConfig>/,/<\/elasticConfig>/s,<enabled>.+</enabled>,<enabled>false</enabled>," file.xml