Were you doing a all-grain or partial mash? If so, differences in brewhouse efficiency (i.e. how much sugar you manage to get out of the grains) can make a difference in OG.
If you were only 'a little' low, I would just leave it. You'll have a slightly lighter beer, but no worries. Perhaps it will even be better that way...that's part of the fun of homebrewing.
I would recommend against it, but adding sugar now would be possible, though you do risk getting a stuck fermentation at five days in. Probably won't happen, but it is possible. If it's still fermenting somewhat it would probably be fine; just don't add a ton, and make sure what you do add has been sanitized by boiling it a while. Chill it again before adding it to the fermenter, no sense in killing yeast by pouring boiling hot sugar on them...
So far as what sugar to use; dextrose will bump gravity (and final alcohol) up a bit, but won't add any flavor. Same with priming sugar. If that's what you want to do, it will work fine. If you actually want to add more flavor to the brew, use a bit of dry malt extract.
I've detailed this a bit in case you want to go ahead with this. Again, it is possible, but again, I think it's unecessary; beer with a slightly lower OG is hardly a tragedy. Homebrewers routinely ferment things with slightly 'off' gravity levels, and make fine beer nonetheless. Getting exactly the right OG is only important if you want to perfectly replicate the results of a previous batch, or a batch someone else made.
RDWHAHB.