You can use ConvertFormula
method.
4th Parameter determines whether its absolute or not. 1 sets it to absolute and 4 sets it to relative. As per one comment to this answer, if you are looking for mixed references, then its bit complex. but reading your question and comments, I think that's not what you are after.
Examples:
'/ Set it to absolute
ActiveCell.Formula = Application.ConvertFormula(ActiveCell.Formula, xlA1, xlA1, 1)
'/ Set it to relative
ActiveCell.Formula = Application.ConvertFormula(ActiveCell.Formula, xlA1, xlA1, 4)
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