Well,the O.P. will be well confused by now. His user manual will describe how to remove/replace the focus screen.
David has the gist of it ,but I 'll repeat : the Thomas Tomosy method,which has the additional benefit of actually working,should replace most of the dubious advice offered so far.
Dilute a little dishwash detergent in warm water. Plunge the screen into this.
Use a very clean,soft brush to brush both sides. Remove screen, rinse in clean water. BLOW DRY - DO NOT WIPE - with anything -no tissue,no Q-tips,Nada.
Lean on tissue to air-dry.
Works on ground glass,acrylic,anything.
One last tip - if the crud is out of focus,but still visible -it is probably on the screen side of the prism. Best method is a small blob of -here it's called Blu-Tak, held in a fine pair of tweezers -it will lift the specks without marring the glass surface.
Now, can we have another thread about how impossible it is to clean a front-silvered mirror ?