How to implement upgrade for fields
You should be aware that once there is data In the tables created for your fields, it's limited what changes you can make.

Anyways want you need to do, is to use hook_update_N in your module's install file. The number N should consist of 4 numbers


Major release of the module (the drupal version)
Minor release of your module.
Number
Number


3-4 is just a combined number that should increment for each time you need to use the hook.

The field module provides a host of functions to add, change and remove fields. You can use those to do what you need.