You can't change the term of it.
You should seriously review whether the policy still suits your needs. It may be that you have paid all the initial charges already and it may be worth keeping on.
If there is a half decent cash value attaching to it now, you might be better off just cashing it.
Check the literature you got at the start of the mortgage. Was it suited to your needs? What sort of fact find did they do? The problems with endowment mortgages were well known 5 years ago, and so they really would have had to make a good written case to show it was suitable. If it was not suitable, then you may have a case for mis selling.
If there was any mention of you paying off your 25 year mortgage within 5 years, then a 25 year endowment should not have been sold to you.
As a matter of interest, what company is it with? Can you provide the figures? What you contributed to date? What is its current encashment value?
Brendan