If you are on maintenance you can install multiple versions of SolidWorks on your machine.
Are you trying to load SWX 2016 on your computer to have both SWX 2016 & 2018 on the same computer? If yes, then do as Carrie Ives suggests. You can install an earlier version (or multiple versions) on the same computer if you have the software and you have (or had) a valid subscription at the time the newest version (& SP) that you want to install was released.
Do you already have SWX 2016 installed and a drawing or part from SWX2018 that you want to view or edit and can't because you don't have 2018 installed? In this case Follow Glenn Schroeders link to see what is happening.
Do you have 2018 and are trying to save the drawing or part to an earlier version (2016)? This is not possible with SolidWorks. However, there are 3rd party software's that can do this providing you have both 2018 & 2016 versions on the same computer and use that software to convert the files.
The only other way is to export the part as a solid part (parasolid or similar) and then open it in the earlier 2016 version as a solid part.
Your question does not tell us exactly what you are trying to do and what software you have.