If you don't have a river or a boat handy...

Get yourself a good quality ball bearing swivel.

Take the leader off the fly line and put the swivel in it's place.

Use a piece of wire, string, whatever works, to attach the end of the swivel opposite your line to something sturdy. I always use a concrete block that I place in the corner of the front yard.

Hold the reel in your hands and back away from the from the tied down end, keeping tension on the line as it pulls off the reel. You'll see the swivel turn as the line untwists. Once you get past all the twists, stretch the line a bit until the swivel stops turning. Give the line another good stretching. And another.

That should fix it.

If not, time for a new line.

Good Luck!