As you know water, electronics and electricity are not a good mix.
The water causes corrosion and provides circuit paths for the electricity which were not in the phone's operating design and could damage the components. Also rice is good to eat but does nothing to overcome the problems caused by corrosion of electronic components due to contact with liquids.
First you need to remove the battery from the phone.
Here is a link that shows how to do that.
Then you need to clean and dry the electronics using Isopropyl Alcohol 90%+ to remove all traces of corrosion and water.
Here is a link that, in general, describes the process.
Electronics Water Damage
You will probably have to replace the battery as it is very hard to recover a battery from water damage.
As always with electronics, especially surface mounted PCBs be gentle when handling and especially when brushing away the corrosion. You do not want to remove any components from the board.
Hopefully after you have done all this the phone might possibly work again