How about replacing “E” with the string "Part " and removing any leading zero from what’s following “E”? … you may read up about the replacement operators of Powershell.
Are the numbers always at the end? If so, this should work:
"Something2 new work [Wake up] S01E01" -replace 'S\d{2}\s?E[0]?([1-9]$|[1-9][0-9]$)','Part $1'
Something2 new work [Wake up] Part 1
"Something2 new work [Wake up] S01 E23" -replace 'S\d{2}\s?E[0]?([1-9]$|[1-9][0-9]$)','Part $1'
Something2 new work [Wake up] Part 23
Break Down:
S\d{2} matches S followed by 2 digits \s? matches an optional space E[0]? matches E followed by an optional zero ([1-9]$|[1-9][0-9]$) this is a capture group matching 1-9 or 10-99
By using a capture group, we can refer to it in the second part of the replace. We will get two matches $0 will refer to the full match (S01E01), $1 will refer to the first capture group (just the episode number).