In each product category you have the possibility to change the sort order of the products. By default Magento sorts the products based on "Position". The lower the number (0 is the lowest), the higher the product will appear.
Log in to the backend of Magento
Go to Catalog > Inventory > Categories
Select the category where you want to change the sort order on the left side
Scroll to the header "Products in Category" and click on it --> the products now appear
In the right column you will find "Position"
Change the positions you would like to change and save the category by clicking Save in the right upper corner