I agree with Wojtek.
Another method, which I've used mainly with success ( the odd small area being damaged, but mainly useable ), is as follows.
Lay the decal sheet flat, on a piece of tissue or kitchen towel.
Using a medium-sized, soft paint brush ( a No.3 is good ), dip this into luke-warm water, and very carefully brush around any edges of the cover tissue which are not stuck, with the brush on the surface of the tissue, slowly and carefully lifting away the tissue. Avoid "flooding" or over-soaking the area - just make it damp, so that the tissue either starts to dissolve, or starts to separate from the decal backing sheet.
Once the tissue starts to separate, it should be possible to remove fairly large areas, by repeating the above, and allowing the tip of the brush to get under the tissue in order to help lift the tissue, although some small "knots" may remain.
To remove these, use the brush, just dampened with water, and progressively move this over the affected area(s), when the remaining tissue should roll into balls and dissolve.
The decals may possibly start to lift very slightly, but leave this, and let it dry naturally, when they should then settle back onto the backing paper.
Also, if the decals have turned really brittle with age, any cracking will probably be apparent at this stage.
Once satisfied that the decal sheet has fully dried (it may be slightly wrinkled, but should be fine - just don't soak the sheet when removing the tissue.), any remaining "dust" from the tissue can be removed by carefully wiping with a soft cloth..
Use an unwanted decal, for example, the "name plate", to test for cracking etc.
If this is OK, then the decals are probably usable, when any remaining small "dust" areas will dissolve away in the water when soaking the decal for use, and can be easily wiped off once the decal is in place.
However, if the decal sheet is now clean, after removing the tissue as described, and once fully dry, then a coat of clear gloss varnish (not "Future"), preferably sprayed, will seal the decals and allow them to be used as normal. I keep a spray can of gloss artist's varnish just for this purpose, used for any old decals which could cause problems.