Because someone somewhere has used the template, so changes are not allowed to ensure data integrity (for example, you could change an answer type that would then negate all previous responses – this would make no sense).
However, all is not lost.
When you click on the title of the template you wish to edit, and opened the sub menu to allow you to edit, you are in fact presented with three choices – Edit, Duplicate, Archive
Click on Duplicate
The system will create a duplicate of the selected template, and will place you into the template generator form, so you can make the chnages you see fit.
Note that by default the name of the new template will have the “(copy)” added to the original template name – you may want to change this but do NOT use the same name as the original if it is still in use as this will cause confusion to the user base