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أين أســـــــــــــتطيع تغيير حفاض الطفــل

In this context "تغيير" is simply a noun not a verb, so it's correct.
Also what you mentioned is correct
so, i change, أغيير
she change تغيير
The word "تغيير" can be a noun as well as a verb.

I hope you are not confused.

to convert verb to present tense ( in Arabic ) you have four characters to insert before , it is (ن-أ-ي-ت) but every one of it have condition to use :
notice : we will use ( gone - ذهب ) to convert it into present
( ن ) for speak men and women ex: we go - نحن نذهب
( أ ) for speak man and woman ex : I go - انا اذهب
( ي ) for he or she and they collect ex : " he go - هو يذهب" " she go - هى يذهبن"
( ت ) for conversation like "you go - انت تذهب" "she go - هى تذهب" " they go - (male)هم يذهبنون" "they go - (Feminine)هم يذهبن"
I hope you understand :)