Address labels make a huge difference in value. This is very important to disclose to prospective buyers. Blacking out a mailing label sticker/adhesive can decrease the value (only to collectors who are picky). Whereas, blacking out a print inset mailing label (where the publisher leaves a blank area for the mailing address to be printed/stamped in by a machine/stamp instead of a sticker – this is a newer way of labelling) is seen as damage to the cover and decreases the value of the magazine.
I have some magazines that are over 30 years old. The way I removed the labels was with a hot air hair dryer. Be patient. Blow the hot air onto the label for at least 2 minutes, very closely but carefully. Then, as the dryer is still blowing on it, try to peel one corner of the label very slowly. If the corner is ripping or hard to lift ..try a different corner. If the label is not lifting, stop. Put more heat on it. And repeat. Much of the time there will be glue residue left over but you can remove that with a nylon (like ladies nylons) very carefully. You stick your finger into a piece of the nylon and rub while holding down the paper cover very tightly to ensure the paper won’t stick (if the cover is thin, I’d advise against this). If the cover is more glossy (without any cracks/rips where the label was), you can use Goo Gone cleaner to remove the residue.
It really depends on the type of magazine cover ..is it papery or more glossy? If it’s more glossy, the above procedure will work really well. If it’s more papery, you need to take much more care and patience (do NOT use Goo Gone on papery magazine covers, it will ruin it).
I’ve heard of Un-du Adhesive Remover, the product @Blueroses mentioned ..you can try that too. I haven’t had any new old magazines in the past few years to have reason to try it out. If possible, try it on another old magazine that you don’t care about first.
As for steaming, like @XOIIO mentioned, ..this is hard to do properly (one can easily damage a magazine, especially when they are not familiar with the process) and not usually recommended for old magazines that are collectables. If you want to try it, place a piece of fabric over the cover. Then, with a clothes iron (on low-steam) go over the label several times. Keep checking the magazine and label. Try to remove the label very slowly. Continue ironing if the label isn’t coming off easily. Patience is important. I have rarely used this method.
The way I do it is not without flaws. But it has worked quite well for me and I’m one of the “picky” collectors :)