When you want to define some transaction instance based on another template (not the default one), you should open the transaction, and in the WorkWithPlus tab, right click on the main node, 'Change Selected Template'.
This also applies when you have already created a Web Panel based on some template and then you want to change the template in which the Web Panel is based on.
It is important to know that when you change the selected template, the instance will be created from scratch. So, all the customizations made in that instance will be overwritten.
And then you have to choose another one and press Select: