The 159 focus screen is not considered user replaceable despite it being fairly easy to remove. The problem is refitting it. In the user manual it says the screen should only be replaced at a service centre and with good reason. If not done correctly the screen will, almost certainly, be damaged. I know, I've done it.
I experimented with an old screen to see if I could find a way the screen could be changed without damaging it. Described here is my method found through that experimentation, it's not a manufacturer recommended way.
Before proceeding a method of holding the screen without damaging it is necessary. Later camera models had screens with a tab to be used to hold the screen by but the 159 screen doesn't have a tab. I use a pair of flat tipped tweezers that have had the tips covered in heat shrink cable insulation. This is permanent and I only use them for this purpose but a temporary solution could be some rubber sleeving fitted over the end of some tweezers.
I also recommend the mirror buffer foam be removed. This makes it easy to see the release catch.
First, the camera needs to held inverted.
A piece of card needs to be cut and bent to fit into the camera such that the edge sits against the back of the focus screen frame (see arrow in next picture). The card will stop the frame jumping out of its retaining slot and will also protect the mirror from possible scratching.
Release the screen by pulling back on the catch. The screen frame will pop up. Lift it up and remove the screen from underneath. This is best done by using something with a point to press against the edge of the screen and lift it up then use the covered tweezers to grab the screen and remove it while holding the frame clear. Don't try and remove the frame first, you will probably scratch the screen while trying to remove the frame.
If you do remove the frame after removing the screen, note it has two tabs on it, a wide one on one side and a narrow one on the other, The wide ones goes to the back, the narrow one to the front.
Refitting is the reverse order to the above. Fit the frame first, hold it clear and fit the screen under it. Note the dull (matte) side of the screen goes towards the prism.
Finally, lower the frame but DO NOT try and press down on it to engage it with the latch.
Use a pair of tweezers to pull the latch back as far as it will go. Use the tweezers against the mirror box frame and make sure the latch is held firmly before going further.
This is the most important part of the process. If the latch isn't held back the frame, which is quite flimsy, will get pushed inwards and will scratch the surface of the screen.
With the latch held back, hold the frame into position then release the latch.
This is not an easy procedure to carry out and requires practice of using two pairs of tweezers and a steady hand.
