as Jimako mentioned THAT file in your theme, do NOT touch or change the 'real' core file ( so only changes are made to the copied template. Please do note, that the changed template due to existance in THAT theme, only works in THAT theme. Should you change in time the theme, do so equal with the template.
Info : In case you wonder why copy : inside theme it overrides the core.... (core file may be changed with each update). SO in case there would be massive changes to it, than look inside it again if those changes should be made in your own changed file too. Success