I am guessing that you are currently on the TRS19 SP5 beta stream and  want to go back to the non-beta SP4 (retail release) stream.
In the Trainz Launcher --> Trainz Settings --> Install Tab --> Change Trainz update stream  then select the stream appropriate to your version. BUT this will only  stop you getting any more beta updates, I don't believe it will turn  your SP5 beta back into an SP4 retail version (I have never reversed the  process so I don't know).
However, it is likely that the very  next release you receive (beta or not) will be the new retail version of  TRS19 SP5 so I don't think you will gain anything at this point except  you will not start receiving the TRS19 SP6 beta releases when/if they  start rolling out.
My theories.