Changing, deleting, moving, etc. of the different objects in the JMP output is one of the most powerful abilities in JMP. This capability is detailed in the Scripting Guide, in the Section on Display Trees. Below is a simple example of doing what you indicated your current need is:
names default to here(1);
dt=open("$SAMPLE_DATA/big class.jmp");
biv = dt << Graph Builder(
Size( 528, 456 ),
Show Control Panel( 0 ),
Variables( X( :weight ), Y( :height ) ),
Elements( Points( X, Y, Legend( 6 ) ), Smoother( X, Y, Legend( 7 ) ) )
);
report(biv)[Outlinebox(1)] << set title("This is my title");
report(biv) << journal;
biv << close window;
biv = dt << Graph Builder(
Size( 528, 450 ),
Show Control Panel( 0 ),
Variables( X( :sex ), Y( :height ) ),
Elements( Box Plot( X, Y, Legend( 4 ) ) )
);
report(biv)[Outlinebox(1)] << set title("A different title");
report(biv) << journal;
biv << close window;
jour = current journal();
jour << Save Presentation("$TEMP/MyTitles.pptx");
open("$TEMP/MyTitles.pptx");
Jim