You're requesting a "formula." Normally, you would enter that into a cell. In such a situation, you might enter the following formula in cell B1 if you have the list of terms in the A column:
=IF(LEFT(A1, 1)="R", "Resolved", A1)
If the first letter in A1 is A, "Resolved" will be added to B1, otherwise, A1 will be copied.
Now that column A is hidden, you can copy this equation down by double-clicking the tiny square in the bottom right corner of the cell. You might also copy column B in its whole, "paste special... Values only," onto column A, then remove column A.