Stephane, what do ou exactly see that you do not like?
If your 'flex does not focus where it should, the problem is not so serious. The focus screen or the viewing lens might be out of alignment.
If the flex focuses in the iddle but the focus is off towards the edges, then indeed the lens board might be knocked out on one side i.e. it is not parallel with the film plane anymore. This is a very serious problem, something the previous replies are referring to. Even a pro repairman has to mess around with this to repair it, and since it is a T model, might be cheaper-easier to get another one...
The adjusment of focus is not a big job, if there is no damage on the front of the camera and lens board is still parallel with film plane. But you need to get access to the viewing lens, as described above, by removing those screws, etc. In any case if you want to get a great working flex, you better leave it to a professional.