The outer joint (tin can) can't be opened or serviced. It's possible to wash it out with solvent, but not possible to properly grease it afterwards. The best you can do is leave the old grease alone and squeeze a tube of CV lube into the can. Let it redistribute itself by centrifugal force. Best boots for this are Astoria boots. They are made of silicone rubber and very stretchy, and come with good clips. Since you can remove the inner CV, there's no need for boot expanders. Factory boots are thick rubber and tough to work with, and the clips don't always fit well. And they cost a king's ransom.

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Based on the rust on that upright, I'd bet you have more problems than just the axle. Now is the time to address anything that needs addressing.