How can I update a column with a value that is the difference (in percent) between two rows of another column?

from:

```
 y | coeff | decreased %
---|-------|------------
15 | 1.35  |
16 | 1.22  |
17 | 1.14  |
18 | 1     |

```

to:

```
 y | coeff | decreased %
---|-------|------------
15 | 1.35  |
16 | 1.22  |  9.629
17 | 1.14  |  6.557
18 | 1     | 12.280

```