How do you expect anyone to answer that?
What kind of file? Text? Word DOC? Excel, Database, Picture, video, executable, DLL, MP3, DWG, OldUncleTomCobley, or propriartory?
How do you want to display it? Text? Binary, Formatted, Picture, Mayan Knotted rope, Egyptian Hieroglyphics?
You need to try to give enough information that at least we can make an informed response - even if we can't help.
And it would help your cause if you showed that you had at least tried to do some of it yourself...
"it's a .txt file & want to display in text format"
So, the first thing to do is to decide how you will decide which part to display.
If you want whole lines, displaying (say) lines 100 - 200 only? If so then:
string[] lines = Files.ReadAllLines(path);
int startLine = 100;
int endLine = 200;
int count = endLine - startLine + 1;
string[] viewThese = new string[count];
Array.Copy(lines, startLine, viewThese, 0, count);
MyTextBox.Lines = viewThese;
If you don't want whole lines, then the principle is the same, but you have to do a bit more work.