Best thing to do. Bottom hose undo as some radiator taps at the bottom decide to snap if having to be persuaded into moving. Let everything drain out. Make sure u have radiator cap off when doing it to make water leave faster, then throw about 2l of water into it at its current state, to get any funky bits out of the rad, i doubt your engine will be filled with much gunk, nothing to worry about. Bottom hose back on, tightened, then put in specific amount of coolant/water mix as suggested by the manufacturer, don't worry about that red coolant, pretty pathetic. Use industry standard radiator coolant for the age of the vehicle. I work at a radiator shop and this is what we tell some people if they can't afford to get it done by us at the time. requires not much time and not much effort.
Once the coolant is replaced, let the car idle for around 15 minutes, occasionally revving it to make sure pressure and temperature will stay at a consistent level.
Hope this helps in some way