To change your ATC, you can create or edit your Income Category in the Category View.

Step 1: Click on the Add New button and select "Income Category"

Step 2: Select the appropriate ATC Code

Most Percentage Tax Filers would fall under PT010 (which Taxumo assigned by default), but you are free to select a different ATC from the list presented.

Step 3: Select the "Income Type"

Your income will most likely come from Sales. In that case, select Sales.
If it is income from a partnership, select Partnership.
If this income does not come from your regular operations, select "Other Income."

Step 4: Press SAVE.

And you're done.

Your new category is now ready to be used:

When creating your 2551Q, the entries tagged against this new category will map to the selected ATC.