Templates are updated and changed through the Designer tool. To open this tool, you user account must have the Designer role. If you have access, then the first step is to go to the Template Management page. Select the template you wish to update or edit. Then choose Designer -> Edit.
This will open the template in Designer, and you can begin to make updates.
IMPORTANT NOTE: Making changes to templates that are in use have implications that must be thought through. In some instances, instead of updating the template, you may want to create a new version of the template instead.