Need to reset admin password with OS X 10.6.8 Will this impact Time Machine backup?
Resetting the password should not do anything to the files stored by Time Machine. Files on the system are associated with a user ID, and changing or resetting a password does not alter the user ID. 

If you delete the account that holds the files you are interested in, this will not directly delete the matching files in the Time Machine backups, but as Time Machine continues to run and throw out old backups, eventually the files deleted from the computer will be removed from the Time Machine backups. But as mentioned, just changing the password of a user account (even an administrator account) does not change the documents made by that user.