What Physics are you using in COMSOL for modeling that? Are you using "Reaction Engineering"? If this is the case, then you find the pressure under the "Mixture" box. You have to change the Reactor Pressure from being calculated using the ideal gas law to "User defined", where you input 50 [bar]. 

I am working with FEM Simulation for PCF structures . Please try with "parameter"section.

Before we start the simulation we should define the parameter. so you can define the initial value of parameter as 50 [bar]

Please have a look about this link: http://cn.comsol.com/community/forums/general/thread/14042/

thank you dear udaya

I know, I define a parameter, but I dont know  where can  I use this parameter? 

I have some experience about frequency mode analysis. First I need to know,

1.       What type of COMSOL version you are using? (I am working with COMSOL 5)

2.       What is the physics that you are using? (I have send the image with this as an attachment )

3. Then we can discuss where to use the parameter. For my research I use parameter

i) to define wavelength

ii)to sweep the wavelength

ii) to define the geometry of objects