== 3M Adhesive Tape from a Phone Parts/Repair Supplier==
This is the only option and is available in many sizes. The glass touchscreen on majority of phones/tablets do not come in direct contact with the actual LCD, it is connected via ribbon connector.
Most replacement touchscreens have this already cut to size/shape and applied but some do not and must be purchased separately.
Samsung, many iPhones and few other brands do have the touchscreen adhered directly to the LCD.
For Samsung phones you have to use a UV activated liquid adhesive but it must dry crystal clear and you do need to purchase a UV light to activate the glue.
I have no idea what type of adhesive Apple products use but I am sure it would cost a lot and you probably will need a vacuum type screen seperator machine.
It might just make more sense to have a reputable and professional repair place do this for you.
If your device is cost effective to repair, you are familiar with delicate electronics and have patience then I would try it but you have to purchase screen repair tools from someplace such as eBay that sells these sets for cheap.
Either way you have to do a fair amount of research before you begin, otherwise it will be nothing but a waste of time and money.