I had to re-learn Creo for a short project two years ago. I found a bunch of 2d-to-3D exercises on-line and plowed through them. When those were done, I found more.
Sketching is the foundation. Work hardest on that.
Every time you make a new feature, explore every checkbox, drop-down, etc. to get familiar with all the options.
Avoid defaults and automatic things at first. Get familiar with all the buttons and switches.