Changing combi mate crystals?

[noclue]

I've been told by some guy at British Gas I need to change my combi mate crystals when he checked out my boiler. Tried to Google this and couldn't find out any information how to do this. I bought some from Plumbase



I have something like this under my sink -



I removed the cover but can't work out how you are supposed to replenish them. What do I need to do? Do I need to turn the water off? Should I just get a plumber to come round and do it?

 

[glenno1]

Google combimate user guide/instructions

 

[jtsplumbing]

When you pull off cover that turn's off water, unscrew glass dome clean out old rubbish, have a look in bottle of replacements you used to get a new rubber seal, screw dome back on and refill through top, push cover back on this will turn water back on.