You'd have to replace the hires video mode with the one you want and recompile the project.
# grep Lb_SCREEN_MODE ./src/*
./src/bflib_fmvids.c: if (lbDisplay.ScreenMode != Lb_SCREEN_MODE_320_200_8)
./src/bflib_test_poly.c: if (MockScreenSetupAnyMode(Lb_SCREEN_MODE_640_480_8, 640, 480, pal) != Lb_SUCCESS) {
./src/game.c: setup_screen_mode(Lb_SCREEN_MODE_320_200_8);
./src/game.c: setup_screen_mode(Lb_SCREEN_MODE_640_480_8);
./src/game.c: LbScreenSetup(Lb_SCREEN_MODE_320_200_8, 320, 200, display_palette);
./src/game.c: setup_screen_mode(Lb_SCREEN_MODE_320_200_8);
./src/game.c: if (lbDisplay.ScreenMode == Lb_SCREEN_MODE_320_200_8)
./src/game.c: nmode = Lb_SCREEN_MODE_640_480_8;
./src/game.c: nmode = Lb_SCREEN_MODE_320_200_8;
./src/game.c: lbDisplay.ScreenMode = Lb_SCREEN_MODE_320_200_8;
./src/game.c: lbDisplay.ScreenMode = Lb_SCREEN_MODE_640_480_8;
./src/game.c: setup_screen_mode(Lb_SCREEN_MODE_320_200_8);
./src/game.c: if (lbDisplay.ScreenMode != Lb_SCREEN_MODE_640_480_8)
./src/game.c: LbScreenSetup(Lb_SCREEN_MODE_640_480_8, 640, 480, display_palette);
./src/game.c: if (lbDisplay.ScreenMode == Lb_SCREEN_MODE_320_200_8)
./src/game.c: if (lbDisplay.ScreenMode == Lb_SCREEN_MODE_320_200_8)
./src/main.c: lbDisplay.ScreenMode = Lb_SCREEN_MODE_640_480_8;
./src/main.c: lbDisplay.ScreenMode = Lb_SCREEN_MODE_320_200_8;
List of modes is within bflibrary. You can also define a new ones there.