Both these operations are done in the Finder.
The templates are found in your User Library. Note that your Mac maintains a system Library, and a separate user Library for each account registered on the computer. It's important to get to the right one!
Open a new Finder window, then press and hold the Option key as you go to the Go menu and choose Library.
This takes you to your User Library.
In the Library, lcate and open each item on this pate:
Library > Application Support > iWork > Numbers > Templates > My Templates
In the My Templates folder, you will find all of the templates you have saved.
To move a Template to the Trash, Click and drag it to the Trash (at the right end of the Dock) and drop it in, OR
Select the file, then go File > Move to Trash, OR
Select the file, then press command-delete.
To rename a Template file, click on the file name twice to highlight the name.
Type the new name.