How can I erase the security copies on my mac?

Hello,

i´m seeking for help about the following.

I do the Time machine dialy on an external Hard Drive.

I´ve must had done something wrong, because suddenly my Mac did the security copies on the internal Hard Drive and I cannot erase them.

Can somebody please help me on this one?

I know it´s something really easy, but i´ve lost :-(

Thanks a lot

Walter