I have some of these and changing the solution in them can be a PITA unless you know an easy way to do it. Trying to remove the clear dome from the indicator body is hard sometimes. Even getting it back on is hard too.
This is what I do. I don't even have to take it apart. I have some disposable plastic pipettes. I turn the CO2 indicator upside down and I use one of these pipettes to draw the old fluid out. I add some more known dKH fluid to the indicator to rinse it out with a clean pipette. Then I use the 'dirty' pipette to draw this out too. Then I add 1 mL of the known dKH fluid into the indicator with a clean pipette. Then I add two drops of the indicator fluid through the hole in the bottom that I've been using to remove the solution. You'll find that the Red Sea indicator fluid bottle fits quite well for this. Sometimes you may have to put the clean pipette into the hole to draw a little air out so that the indicator fluid goes in. Then, you place it back into the tank. This only takes about a minute or two to change the solution. It beats the heck out of trying to wrestle the clear dome off and back on.
I keep using these disposable pipettes over and over by keeping them clean. I use 1 mL and 3 mL pipettes for this, but one or the other would work just fine.
I hope this helps some folks.