I am a beginner in Craft CMS. I just want to know, what will happen if we change the field type in the fields (settings -> fields) in Craft 3?
It depends on the field type. Many of them you can change without problems, while others would make no sense. If you are, for instance, changing a text field to an image field, Craft obviously wouldn't know what to do with it. But if you change an Email field to a Plain Text field, it would keep the value.
The dropdown will have a warning icon when it's unsafe to change the field type. Sometimes it may work, even though the results are unpredictable. For instance, if you change a Plain Text field to a Multi-Select field, the text will be the value of the field (I think), but there is no guarantee that it is a valid option for the Multi-Select.
Also, the other way, changing a Number field to a Plain Text field causes no problems. If you change a Plain Text to a Number field, you have no guarantee that the text actually represents a number. But I think it will keep the value, but that you will get a validation error on trying to save it the next time, if the value is not valid.