The mousewheel on my Prime Wireless mouse recently broke - it would "click" but was free-scrolling and had a fair amount of wiggle to left and right, so it effectively just turned into a middle mouse button.
This was a bit annoying, and after taking the mouse apart, I discovered the cause: the wing of the mouse wheel's shaft that goes into the rotary encoder had snapped, so there was now a small piece of plastic wedged inside the encoder, and the mouse wheel was supported just by the outer chassis of the mouse and the interior LED.
First, I tried to repair the existing mousewheel using some plastic glue. Honestly, this might have worked, except my approach was to pop out the tiny bit of plastic from inside the encoder, and then glue it back outside the mouse. This was really a mistake, given what I had available - the plastic part was too small to manipulate well, and I didn't have helping hands or a magnifying glass to help me put things together.
I checked the Steelseries replacement parts website and... they don't sell replacement mouse wheels. Bummer. However, I did find what looked like it might be a compatible mouse wheel from AliExpress. This is for a different SteelSeries mouse, but the main difference appears to be the pattern of the rubber. The Prime Wireless has straight lines cut into it, but this part has a more "tyre-like" pattern.
To avoid being held up by potentially spotty shipping from China, I also just ordered a replacement identical mouse. Hey-ho, at least I'll have a spare if the battery in this one starts to go bad.
After the replacement mouse wheel arrived, I pulled the broken mouse apart: there are four Torx screws on the bottom of the mouse, underneath the PTFE mouse feet. (Note that you might want to also buy replacements for these: they probably won't stick back on very well after being removed.) Once those screws are removed, the mouse can be carefully pried apart. Be careful! There's a generously-long flex cable that attaches the main mouse PCB to the mouse buttons on the upper chassis, and this can be unclipped from the main mouse PCB. Once you have the mouse apart, there are two cross-head screws that hold the mouse LED PCB onto the bottom chassis, which should also be removed. This frees up the mouse wheel, which you can now swap and replace with the replacement. Afterwards, you can reassemble the mouse (don't forget to reattach the flex!)