Let us take a sample pattern as shown below, having four cuts that makes two parts. Note the quantity of the pattern is one.
Now, we will shift the first horizontal cut by 20 mm. So the first cut dimension will be 2100 mm. We will also have to add one more horizontal cut with dimension 2080 mm that will make the part named "1". The remaining cuts will remain as they are. The edited cuts grid is shown below.
When validate button is click, the new pattern is made as below.
Note the first cut is shifted by 20 mm and the number of cuts have increased from 4 to 5.