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