"For frozen caliper, if I take out rear brake caliper and clean it and put some brake grease on it, will it be enough?
I haven't flush brake fluid ever. When I changed brake pads, I see brake fluid is clear and sufficient. Is there any sign for me to flush brake fluid? Can the flush be done DIY?"
"Frozen" caliper means the piston is stuck in its bore, so the caliper must be rebuilt/replaced. "Binding" would describe the caliper not floating/centering on the mounting bolts, which be addressed by cleaning/lubrication as described above by others. Flushing all the brake fluid is recommended every two years, and can simply be done with a helper pushing the pedal while you open the bleed screws, or you can use a pressurized one-man bleeder system. Don't forget to bleed the clutch system if you have a manual transmission; It uses the same brake fluid from the same reservoir.