Each variable have an attribute displayed in the corresponding column of the grid. Physical I/Os are marked as either "Input" or "Output". Inputs are read-only variables. For each internal variable, you can select the "Read Only". Otherwise, the "attribute" column of an internal variable is empty. Parameters of User Defined Function Blocks are marked as either "IN" or "OUT".
To change the attribute of an internal variable, enable the modification mode in the grid and move the cursor to the selected "attribute" cell. Then press ENTER to set or reset the "Read Only" attribute.