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to learn how to post code correctly and other useful tips for newcomers.
Refer to the List API (search for it using your favorite search engine), examine the methods available there to:
1. Find the desired entry in lst
2. Modify it as needed
You don't identify the type of data being stored in lst, but to change the desired element to type Timestamp may require adjustments either inside or outside of lst. Once you get some code, show that and we can be of more help.