I was surprised to find out there are some rechargeable Li coin cells that are indeed 3.6V (full charge) cells. If these are those, you charge them up to 3.6V not 4.2V. If they have 3.6V stamped on them, that could be what they are. If you have the manufacturer's part number, do a net search and make sure which kind they are so you don't overcharge them.
The vast majority of these cells aim for 4.20V as the endpoint. Typically, Li-Ion cells are charged by first applying a constant current (at some fixed value, often 0.5C) until the battery voltage is 4.2V, and then applying constant voltage until the current flow into the cell drops down to about 0.1C at which point the charging is done.