To edit protein side-chain conformations in SAMSON, use the Rotamers editor extension based on Roland Dunbrack’s famed library.
Select a residue, then choose the rotamer you want to use from the corresponding table. Potential clashes will be automatically detected and shown in the viewport.
Get the free extension at https://www.samson-connect.net/element/7cc74ea8-c24b-78fa-49a6-a75562902a12.html.