Get a bucket of water big enough to put the rootball in with room to spare. Get in there with your hands and gently work everything loose. Once the roots are bare and more spread out,fit them into a pot that allows a bit of room to grow but not too much,and refill with fresh new mix while shaking the container a bit for even distribution of mix. Water well and presto...you're done. :)

A photo would help us know what you're up against.

Pull it out of the pot and leave it soaking in a bucket of water. Leave it over night. A philodendron can handle this fine for a day or so.

This should soften the remaining soil in the root ball. Gently squeeze and roll it between your hands and it will start falling apart, Holding it in the water while you do this can be helpful.

Clean off as much of the old mix as possible and cut off all the roots that are soft and mushy. Then re-pot it.

One trick I use is to take the plant out of the pot and blast the rootball with the "jet" setting on my garden hose sprayer. This helps knock away most of the soil and loosen up the roots without damaging them. Then you can go in there with your fingers and gently untangle them even more. As I'm doing that, I use the "shower" setting and run a steady stream of water over them. That helps to untangle them much easier than it would without the water "lubricating" them, so to speak.
I just recently did this with a cute little fern I bought. Its roots were thin and delicate. I couldn't even get my fingers into it just a little bit, to try and loosen them up, without ripping apart its roots. I was even concerned that blasting it with the garden hose would damage them, but it didn't. Now my fern is enjoying its new home and fresh soil.