The link is to the example sheet of what I have done

https://docs.google.com/spreadsheets/d/12w4rGArGi1I1wlpm5yJJtT5AAlMM4LcZC31_DpP6jZQ/edit?usp=sharing

I shifted the rows that contain "PO" in my data selection. (see shift function)

It was suggested to me that I use array functions to made the code better, but I am honestly not sure how to refactor it. I am still very much a beginner.

I'm guessing I can more easily do this instead of trying to reassign array index values.

Here is my current script:

function shift() {
  try{
    var ss = SpreadsheetApp.getActiveSpreadsheet();
    var as = ss.getActiveSheet();                     
    var ar = as.getActiveRange();
    var vals = ar.getValues();
    var r; //variable for rows

    // in this first section, I've already stored the active range (my selection) into a two dimensional array.

    for (r = 0; r < vals.length; r++){                                           // for each row, up to the last row (iterate over all rows from top to bottom)
      if((vals[r][0].indexOf("PO") != -1)||(vals[r][0].indexOf("P0") != -1)){    // if first column in each row contains "PO"
        var c; // variable for columns                                                    
        var cols = []; // array to store all data temporarily (will be uses to set new values later)
        for (c = 0; c < vals[r].length; c++){                                   // then iterate over each column(cell) in the row
          if(c == 0){                                                           // if it is the first row,
            cols[c+1] = vals[r][c];                                             // assign second index of the array with the PO value (to simulate a shift)
            cols[c] = "";                                                       // assign the first index of the array a blank string
          }
          else{                                                                 // if it is not the first row
            cols[c+1] = vals[r][c];                                             // assign each additional column value to the next index (+1) of the array
          }
        }
        for (c = 0; c < vals[r].length; c++){                                   // once the array is finished, loop through the columns again foreach row
          vals[r][c] = cols[c];                                                 // this time, assigning the new values to the corresponding array indices
        }
      }
    }
    ar.setValues(vals);                                                         // now, set the values that you reassinged to the array
  }
  catch(err){
    SpreadsheetApp.getUi().alert(err);
  }
}