For many reasons you may wish to change a component in your design, whether it be a specification change to the design, a supply issue for the part you designed in or you find that you have existing stock of a similar specified component. The method is the same.
Here we have a simple example with two diodes.
You decide that D1 the 1N5817 will be replaced by a STTH104A
Remember that DesignSpark PCB works from the schematic to the PCB, so select D1 in the schematic and right click and select the "Properties", now select the "Component" tab and next to the "Component" there is a "Change" button, simply click that to reveal the libraries and components you have available.
You can now search by the library for the component you wish D1 to be replaced by.
Here you see by selecting the DesignSpark library and using the pull down list (green arrow) the STTH102A is selected. Click "OK" and the component is added to the design for D1.
Now we have D1 updated in the schematic and need to update the PCB to complete the process.
Forward the "Design Changes" and the PCB will update. A message box will appear to allow the component to have a different package, leave that checked and click "OK".
The PCB is now updated with the component and the tracks to accommodate the change. A report is displayed to identify what is changing and the PCB layout has the new footprint.
Depending on the change of component footprint, the adjacent component and tracks there may be some additional rework required.
Note: This example uses images from V8.1