The alternative characters (or glyphs) will be controlled by specific OpenType features. Most likely either ligatures, stylistic alternates or contextual alternates. How you control the use of these features will depend on the software you are using your font with.
For example, in Illustrator you can control which features are used in your text through the OpenType panel:
Or you can manually insert specific characters using the Glyphs panel:
If you want to know how to control these features in the program you are using simply search the documentation for that program for "OpenType features".