El Capitan upgrade issue
I've never known Time Machine to hold up an OS update, however…

System Prefs > Time Machine [or from the menu bar if you have set it to show there] you can tell Time Machine to halt its current backup, or switch it off completely.



A late note: if you have local snapshots enabled [which I don't, in the picture above] Time Machine changes its "Time Machine keeps:" line to
"Time Machine keeps local snapshots as space permits, and:"