How do I delete files from a timemachine backup

So my mac was acting very slow and so I backed up all my files to an external hard drive using time machine and then proceeded to restart the entire mac. Now that I plug in my hard drive with my back up I dont really want to put it all back since my computer is running so much faster. My question is then how do i delete some files on that back up. Every time i press delete it says files can not be deleted from this back up time machine?

You need to 'Enter Time Machine' by clicking on the menu bar icon that looks like a clock face. One of the options is to enter time machine. This transforms your screen temporarily into a time machine window. From there you can select and delete time machine files.

A straight deletion will only get rid of the file selected in the individual backup you're addressing. However, when in Time Machine, right-clicking on a file will show an option to delete all instances of the selected file in the current and other Time Machine backups.