There are two functions around reassigning donations. They are, reassigning one donation or reassigning all donations belonging to a donor.
To learn how to execute both of these functions, please watch the video below.
If you would like to merge a contact, use the reassign all donations button. Then, copy any data from the record to be merged you would like to the profile you would like to use for this contact moving forward. Then, delete the record you moved the donations and information away from.