Open your PP3WorkProfile.ini file in C:\Users\USERNAME\Documents\My Games\PP3\PP3WorkGame\Config
Change all instances of NumCompleted=0 to NumCompleted=4
Change all instances of ECT_NOT_SUCCESSFUL to ECT_FIRST
On line 8, change the value of Rating= to Rating=9999
Look for any instances of NumCompleted= and make sure they're all set to NumCompleted=4
Look for any instances of ECT_ and make sure they're all set to ECT_FIRST

Navigate to the following directory in File Explorer.

C:\Users\USERNAME\Documents\My Games\PP3\PP3WorkGame\Config

Make a copy of the PP3WorkProfile.ini file in case you screw something up.

Open the original PP3WorkProfile.ini in Notepad. Turn on Word Wrap if it's not turned on yet, it's under Format -> Word Wrap.

In Notepad, navigate to Edit - > Replace...
In the Find what: field type the following.
NumCompleted=0
In the Replace with: field type the following.
NumCompleted=4
Click Replace All

Now we're going to do the same thing, but with a different string to replace.

In the Find what: field type the following
ECT_NOT_SUCCESSFUL
In the Replace with: field type the following
ECT_FIRST

Now you need to change your rating
In Notepad, navigate to Edit - > Find...
Type Rating in the Find what: field
Change whatever your rating is to 9999
The string should end up looking like this
[Rating=9999]

We're still not done. If you already unlocked some content you need to manually change some strings. Open Edit - > Find... again and find NumCompleted Work your way down the strings Notepad finds, and see if any of them are at anything besides NumCompleted=4 If they are, change them to NumCompleted=4. NOTE: DON'T CHANGE NumCompletedRaces OR NumBonusRaces

Again, open Edit - > Find... and this time find ECT_NOT_SUCCESSFUL. If you get no results, you're fine. If you get any results, change them to [ECT_FIRST[/i]

Launch the game and enjoy.

Original credits to this guy whose instructions were fragmented into two videos, which is why I wrote this up.