The lens in hand is a PKA mount sigma superwide II 24mm f2.8, however the sigma miniwide II 28mm's are structurally very similar and my guess is that they will adjust in the same way.
The reason why you might need to do this is because the focus barrel is held in place by the friction of a screwed tight plate. This means it is very possible for eg a knock to shift things slightly, causing infinity focus to be lost. However fixing this is quite straightfoward.

1. First remove the nameplate. As you can see in pic 1, my friction tool for this is a suitable diameter piece of pvc pipe with some double sided sticky tape over the end. I have found this to be perfectly effective, one small drawback is that there can be small sticky smears on the lens after.
   Do check for filter thread dents first. I didn't notice a small dent on the lens pictured, this didn't affect mounting a filter but did prevent the nameplate coming off. I had to backtrack, ameliorate the dent and then try again.
2. With the nameplate off you can see 3 small JIS screws, the position of two of them is circled in pic 2. Use a 00 size JIS screwdriver to take these out. The top end of the lens barrel now comes off. Note that the positioning of these screws is assymetric so it will only go back in the correct position.
3. This exposes 3 more JIS screws holding an unpainted metal plate SP, pic 3 (the inner silver metal part is the top of the helicoid,). Unscrewing these allows the focus barrel FB to lift off - Pic 4. In pic 5 I have outlined in red the stop tab on the lens T, this runs in the indented part of the focus barrel between T'-T'. H is the top of the helicoid. The unpainted plate grips the focus barrel against H. To adjust the focus, the plate needs to be rotated a bit relative to the focus barrel (perhaps I should say the focus barrel needs to be moved relative to the plate). In pic 4 I have arrowed the direction the plate needs to be moved to correct being short of infinity focus. You may be able to do this by just slackening the 3 screws with the focus barrel in situ on the lens. If you can move the focus barrel without actioning the helicoid then you're in business, proceed as follows:
   - set the focus on infinity.
   - slacken the screws.
   - move the focus barrel back a bit from infinity. Note that the adjustment required will typically be just a few mm.
   - retighten the screws , refocus to infinity and check.
   The fit of the plate on this lens was too sticky to do that so I took the focus barrel right off and used my fingers to push the plate out a bit, move it a bit, then press it back into place (after I did this a couple of times it loosened up). It may well be helpful to make a mark or scratch so you have a reference.
4. At this point checking infinity focus is likely to involve a bit of trial and error to-and-fro-ing. Live view focus magnified, may be good enough. Alternatively, or lacking LV, take the pic of the remote subject and pull it up on the lcd and check focus by enlarging it with the dial. To try to be more accurate would involve taking the pic to the PC to look at it full sized, methodically, and millimetrically readjusting the focus, repeat.
   ATTENTION. Bear in mind if carrying the lens around partially disassembled that the front element is loose and could drop out.
   Once satisfied, reassembly is a simple reversal of these steps.