One of the first, simplest and commonest adjustments tackled in Photoshop. However, here's a method you may like to try which gives you more flexibility and enables you to undo earlier mistakes.
Let's suppose you want to clean-up a spotty skyline:
1) Make a new layer above the one you wish to clone (Ctrl Shift N) Double-click the layer name and call it 'Cloning' or similar
2) Press S to activate the 'Clone Stamp Tool'
3) Make sure 'Use All Layers' is ticked in the Options at the top
4) With the new layer still active, do your cloning to clean-up the sky and, say, remove TV aerials and repair chimney pots.
5) While you're about it, get rid of those yellow lines and move the man carrying the Tesco bag
6) When you've finished, and look closely, you spot some dodgy cloning you did about 10 moves back. Do you go all the way back in History and redo it all? No! Load up the Eraser Tool (press E) take out the dodgy bits then go back to Clone (S) and try again.