I need a new watch band and I cannot take the old one off

If you followed these steps:

Change your Apple Watch band - Apple Support

(especially the 'Learn more' section) and still have problems, your best option may be to take it to an Apple Store and let them look at it. Don't force it. or you may break it.

If the band doesn't disengage then this will need to be looked at by an Apple Store or Authorised Repair Provider. Apples current policy for bands not disengaging is to replace the watch and band together. This is subject to the warranty status so is likely to be chargeable as out of warranty given the length of time you've had the watch.