You can pick the tomatoes and leave them to ripen in the sun. If you put other ripe fruit next to your tomatoes they give of a chemical that speeds up the ripening process.
You can pick the whole bunch keeping them on the stem or singularly.
By picking the fruit you stop the tomatoes from continuing to grow and they then go into ripening mode.
Alternatively you can reflect sunlight onto your plants. It will speed up the ripening process but with reflectors in your garden or allotment, it won't look very pretty.
Not all tomatoes ripen at the same time, so you may want to just leave nature to do its thing.
best wishes
Neil