This is a two step process:
1) Tick “enable any user to set variable” checkbox in the RPD for the session variable
2) When you create a dashboard prompt, in the Set Variable list, choose to populate a variable for the dashboard prompt using a server request variable.
If you set a server variable it will override explicitly the value of this variable set via the initialization block.
The variable is changed in the dashboard that uses the prompt( where the variable is set).