As for lubrication follow instructions provided.
It works for tacho as well.
Try to remove the shaft first, it should give better access to the thumb nut holding the instrument.
The speedo flexible shaft attaches to gearbox at its rear left end. It is possible to remove and install speedo shaft with gearbox in place. Remove the holding screw completely, otherwise you will not be able to remove the housing without breaking it.
If it comes to replacement of the shafts, observe the difference RHD and LHD when ordering.
I am not experienced enough to judge from the noise if it is quick fix (tightening, lubrication), medium fix (shaft replacement) or the big fix (speedo overhaul). In my car I had a case - I had to tighten the speedo shaft as it made noise, but short after that it just snapped, so I had to replace it. Sometimes when the speedo gauge itself is vibrating it may mean it is its internals that need attention - ends often with overhaul.