If the debt has already been sent to collections then paying off the debt in collections will do nothing to improve your credit score.
That is unless you speak to the creditor to see if they'll have it removed from the credit report once the debt that was sent to collection agencies was paid off.
If you can't pay the collection agency in full then it's best to not pay them at all because once you do start making payments the clock starts again at 0 and then it takes that much longer for the collections to stop affecting your credit.
A collection has up to 7 years to collect a debt from you legally and then after the 7 years the debt collector can no longer take you to court or sue you to collect the debt and after that 7 year period as long as you don't speak to the collection agency the collections will no longer effect your credit score.
Only pay collections if you can pay them in full but there's no guarantee it will improve your credit score.
It's best to work the debt out for payment with the creditor before it gets sent to collection agencies and ruins your credit.