You could store all lines in an array, called say "lines", which would have 8 indexes.
>lines[8];
Then, simply have an int that represents the first array index to be displayed (called Top), and another denoting the number of lines to be displayed (ScreenSize). When the users presses up/down, change the value of the Top, and redraw.
Code:
lines[8] = {"line1","line2", "line3", "line4", "line5", "line6", "line7", "line8"};
int Top = 0;
int ScreenSize=7;
DrawScreenLabel:
ClearScreen;
Loop Counter From Top To (Top+ScreenSize)
Printf lines[Counter];
End Loop
If UserInput = UP Top--;
If UserInput = DOWN Top++;
Goto DrawScreenLabel;
A certain amount of validation will be needed to ensure you stick within the array bounds, so be careful! And don't use a goto, like in my pseudo code