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I don't think can get Inventor to show fractions by default but, if it's just the odd 1 or 2 dimensions you can double click the dimension then, in the text tab select 'hide dimension value' & insert any text that you want.
Fractional display is only managed at the style level (i.e. not as an override like precision and tolerance), so you'll likely need to create a new dim style with the desired settings and apply that style to desired dims.
Right Click on said dimension, new dimension style, and change it to fractional, then it will allow you to change individual dimensions either dicimal or fractional. you can to use both styles in your drawing. Good Luck!!!!
Simply left click on the dimension so it remains highlighted, left click on the top right drop down(don't know name, see attached screenshot) and left click on the "Default - Fraction (ANSI)" button. This will change a single dimension to fractional.