The best way is to put a drain tap in when you set the system up but since you didn't do that your safest option is to remove the whole water system intact and drain, flush & refill in the bath or similar place. If you don't want to remove your water cooling components from your case you might be able to drain some coolant out via your T-Line depending on it's position. If you can remove some fluid from there and then find a tube termination lower down that you can bend up so that you have air at the join you can unclip there & then tip down into a bucket. Ideally you'll have your pump at the base of your system and will be able to raise it up slightly to create your air pocket, disconnect your tube from the pump inlet, tip & drain and then power on the pump to get the last bit of coolant out. Hard to do without getting wet though Just be careful and think hard about what you're going to do before you do it.
Of course it goes without saying that you should make sure your PC is totally switched off & unplugged before you do anything.
Well I have a 16 bay server case. So between the two sides is a wal and the tubing runs through the wall at top and bottom. However I did get it takin care of last night. I unscrewed my heatercore and moved it a little and slid the tubing back to allow air in and then removed it.