You firstly need to add the following to your XML file for your module:
<updateservers>
<server type="extension" name="Name of Module" priority="1">http://www.example.com/update.xml</server>
</updateservers>
Then, create a new XML file called update.xml (if you want to call it something else, then be sure to change the name in the code above) and add the following code to it:
<updates>
<update>
<name>Name of Module</name>
<description>description goes here</description>
<element>mod_my_module</element>
<type>module</type>
<version>1.0.0</version>
<downloads>
<downloadurl type="full" format="zip">http://www.example.com/module.zip</downloadurl>
</downloads>
<maintainer>Company Name</maintainer>
<maintainerurl>http://www.example.com</maintainerurl>
<targetplatform name="joomla" version="2.5"/>
<client>0</client>
<client_id>0</client_id>
</update>
<updates>
Every time you want to release an update, you simply need to duplicate the <update>
tags and everything inside it and put it above the previous one. So here would be an example of version 1.0.0 and 1.1.0
<updates>
<update>
<name>Name of Module</name>
<description>description goes here</description>
<element>mod_my_module</element>
<type>module</type>
<version>1.1.0</version>
<downloads>
<downloadurl type="full" format="zip">http://www.example.com/module.zip</downloadurl>
</downloads>
<maintainer>Company Name</maintainer>
<maintainerurl>http://www.example.com</maintainerurl>
<targetplatform name="joomla" version="2.5"/>
<client>0</client>
<client_id>0</client_id>
</update>
<update>
<name>Name of Module</name>
<description>description goes here</description>
<element>mod_my_module</element>
<type>module</type>
<version>1.0.0</version>
<downloads>
<downloadurl type="full" format="zip">http://www.example.com/module.zip</downloadurl>
</downloads>
<maintainer>Company Name</maintainer>
<maintainerurl>http://www.example.com</maintainerurl>
<targetplatform name="joomla" version="2.5"/>
<client>0</client>
<client_id>0</client_id>
</update>
<updates>
Hope this helps