Changing a character in a string

[Larz60+]

looks like a genetic sequence
What have you tried?
We're glad to help, but don't generally write the code for you.
But since this is so simple:

>>> zz = "ATCGATCGATCGATCGACTGACTAGTCATAGCTATGCATGTAGCTACTCGATCGATCGATCGACGATCGATATCGATGCATCGACTACTAT"
>>> zz = zz.replace('T', 'U')
>>> zz
'AUCGAUCGAUCGAUCGACUGACUAGUCAUAGCUAUGCAUGUAGCUACUCGAUCGAUCGAUCGACGAUCGAUAUCGAUGCAUCGACUACUAU'
>>>