Welcome to the forum.
All fills are associated with a Smart Polygon. If a filled area is created from a series of single lines then a Smart Polygon will be generated to contain the fill/pattern.
To change a fill color or pattern you need to select the smart polygon then use the "Fill Properties" icon in the Inspector to make the changes. If a fill region can't be found by the program, try holding down the shift key and select each line forming the desired polygon. Should there be lines that overrun each other then consider using the "Region Trim" tool to clean them up.
If the fills overlap you can use the "Arrange" option in the "Edit" menu to select the order the fills appear.
If this doesn't get you going then post a sample drawing for us to look at.