Depends on what you are trying to do it in. Different apps have different abilities.
If you are wanting to make the change just for a particular text you are trying to create (depending on the app) you would need to just make the change within the window in which you enter the text. Most have this option for a few symbols. Now if you are wanting to do it for the whole app, and just that one app, then you would have to check the apps settings for any keyboard changes it allows.
Lastly, if you are wanting to make the change for the whole device, which would effect everything, you would then need to download and install the right keyboard. A standard keyboard has a limited amount of symbols.