Although rendering a large area is a skilled job, patching is less difficult. However, even in experienced hands, 'invisible' repairs are hard to achieve, so you may need to allow for redecoration of the whole wall.

Can you touch up render?

You do not need to take the old paint of you can seal it with a pva or plaster bond, then you can patch up the render and repaint.

Can you fill render?

Press filler or rendering into the crack with the filling knife. Filler is more convenient than rendering but less economical for more than a couple of cracks. Prod the knife into the cavity to make sure there are no air pockets in it. Smooth the filling level with the wall surface.