In order to change the travel in a Solo Air equipped fork, the Solo Air spring assembly will need to be replaced with the proper length version for the travel length you require. You can find the part number of the Solo Air spring assembly specific to your fork on the RockShox spare parts catalog, found on the service portion of our website.
The part number you will need will depend on your fork model as well as wheel size, as they are specific.
For Dual Air forks, travel is changed by removing or installing an All Travel spacer on the spring assembly. Please see the service portion of the SRAM website for specific instructions for your specific fork model.