Assuming you mean original equipment Girling (or similar pattern replacements, such as Hagon?) shocks?
The spring must be compressed (but without compressing the damper unit) until you have sufficient clearance between the damper end and the spring to extract the (usually slotted) retaining collar.
Proper motorcycle shock spring compressor tools can be bought, however any reasonably resourceful person should be able to make something that will do the job, and should you choose to do so, I suggest you go to Google (or other browser) Images, and search using the term: "motorcycle shock spring compressor" and that should give you some idea of how to go about it.
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