Every Uchiha who takes level 15 can use the sharingan. Evey times that you use this jutsu you will gain a sharingan exp.

After all you need find the uchiha stone inside of clan uchiha, but to upgrade your sharingan you need some amount of sharingan exp. (you can only read this stone if you have the sharingan open.)

You can also remove your sharingan to give to another player, find Itachi and speak with him about eye and then remove. Itachi will remove your actual sharingan and give to you, making your eye back to sharingan level 1.

If you Have the item is just use it and you will upgrad your sharingan.

The sharingan only can be removed if you have this in level 2, 3 or mangekyou.