If the notebook contains separate cells and is not one long sequence of items separated by semicolons then you can click the cursor into any single cell and <shift><enter> will evaluate that cell.
You can divide one cell into two by positioning the cursor between expressions, clicking the mouse and then <ctrl><shift>d to split the cell.
You can highlight part of an expression or a whole expression inside a larger cell by positioning the cursor at one end of the expression you want to evaluate, pressing the left mouse button, and while holding that button down, dragging to the opposite end of the expression you want to evaluate and releasing the mouse. Then <shift><enter> will evaluate only the highlighted expression.
There are perhaps other ways of doing this.