-Flat head screw driver
-Star bit for the fastener, most of these cars have a hex head bolt
-If you don't have a fluid sucker outer, you could siphon it out but I wouldn't recommend that........
Suck out the coolant out after your engine is cooled down-
Remove the bolt securing the tank and use your pliers to slide down the clamp, the hose pulls off very easy-
With the top hose removed, lift up on the tank to get at the bottom hose, take your pliers and pull the metal clip back. Once it's back you can wiggle the tank out of the fitting. Hold the hose up after disconnecting to avoid dumping coolant-
Check to see if you're missing the same grommet I'm missing. There should be a grommet in that hole by the red wire, it's where that little post on the front facing side of the tank is secured. If it's missing, order one- https://eminiparts.com/parts/2012/Mini/Cooper/index.cfm?action=partDetail§ion=COOLING SYSTEM&group=COOLING&subgroup=RADIATOR & COMPONENTS&component=Reservoir grommet&partnumber=17132751855&vehicleid=379810&siteid=217261
Push the metal clip back in and press the fitting onto the new tank. Press until you hear a very noticable click-
Re-mount the tank to it's normal installed position. Re-attach the top hose to the tank. Pop that little hose clip off of the old tank and stick it on your new tank. Fill it to the max full mark and screw the cap back on-
Stand and admire your new non leaking expansion tank-
Prolly takes 10 minutes, if that.