Hi Conrad,
Check out page 29 of the manual https://www.benelliusa.com/sites/default/files/content/media/manuals/2020-12/Lupo Final Manual 02-28-20.pdf
6.Insert the bolt cap into the bolt unit, aligning the marks on the two parts; compress the spring (a) pressing the side button (b) at the same time, then turn the bolt cap counterclockwise (c). NOTE: for this operation, it may be useful to have a support surface.
Essentially, you line up the two red dots, force down, and twist the bolt cap counter-clockwise. It helps to place the bolt face against a hard flat surface while pressing down during the twist.