Rolex has a special tool/jig for opening/closing riveted/folded links that was fairly ubiquitous among professional watchmakers until the late 70s. These days, I believe only some RSCs have them. For us mere mortals, your only real options for riveted links are to grind the rivets off, remove/add links & replace the rivets (good luck finding replacements though), or unbending 2 links with a pair of screwdrivers (or whatever works for you), adding/removing a link & rebending them back. This latter option nearly always, in my experience, results in the bent links' outer skin being left somewhat dimpled or mangled.
For folded links, the latter option is the only option.