How can we edit InDesign templates?

normally templates in InDesign are made using other programs such as photoshop or illustrator, so if you want to edit that template you will have to go back to your psd or ai files and edit them form their.

You can edit it as if you were working on a regular InDesign files or documents, but there might be some elements on the master pages and you can't edit or change their attributes unless you go to the master page or press Ctrl+Shift+Click on the desired element you want to change or edit.
Some elements might be pictures where you might need a program such as photoshop to edited, or vector files such as eps or illustrator and you can edit it in Adobe Illustrator After editing the template you might want to save it as you own personal template by saving it as InDesign file template.
If you can address more details to your problem, I might give you more proper answer.