Custom formulas can be added right away to a normal cell. However, for a predefined formulas in Tabular, you have to remove the existing one, before adding your own formula.
To remove formulas:
1. Click the appropriate formula type you want to remove in Validation section of the ribbon.
2. You can choose to remove the formula from a selected cell or selected QRT.
3. Click Delete formulas.
4. A confirmation message appear that the formula is removed.
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