I have to replace my toilet tank<41 years old) The existing water supply line is metal.
Can I purchase a line at my local home center for replacement purposes,(Flexible line).
If yes, how do I change lines?? I am also replacing the water fill line, and toilet flapper.
Does it matter in what order this is done? Thank you.

Can I purchase a line at my local home center for replacement purposes,(Flexible line).
If yes, how do I change lines?? I
Yes, ask for a toilet supply line. The small nut will connect to the shut off valve and the larger one screws on to the ballcock.

I am also replacing the water fill line, and toilet flapper.
Does it matter in what order this is done? Dealers choice! Install in any order.
Does "water fill line"= ballcock? You install the ballcock by the directions inside the box. A flapper replacement is one the easiest repair jobs going. Let me show you how. Shut the water off at the angle stop located under the tank and flush the water out. If you have a newer toilet you will see where the old flapper hooks onto the base of the white overflow tube. On a older type with no hooks the flapper has a ring that slips down over the overflow tube to the seat. If your tank has hooks, take a sharp knife and cut the neoprene ring off on the marks provided and hook the flapper on the hooks. ( This MUST be done or the flapper will not seat) Now attach the linkage,(chain) to the flush lever. The chain should have 1/4" of play when the flapper's seated. Too much play and the flapper will close too soon. Too little and the will not be allowed to raise high enough. Set your water 1/2" below the white overflow tube and be sure the little tube that comes out of the top of the ballcock is connected to the white overflow tube.
Good luck, Tom