iphone 7 on 76% battery status ‘service’

I have a standard iPhone 7 model - contract for basic calls txt and 5gb just renewed (again) orig from end 2018 runs now to Sept 2024 

Phone works ‘ok’ but noted more frequent charge req’d and discharges quicker. occasional unplanned shut down.

Turned all backgrounds AP’s off to preserve life. General tsb for battery notes 76% max but states it’s sufficient to run phone successfully. 
don’t need any NASA standard gadgets or ‘status symbol’ B/S. 
 

Please some intelligent advice /  thoughts on:

1. replace battery (self or service centre?) OR
2. replace handset for new 
3. is latter possible with 1 off payment or part into contract with monthly fee increase (current only £6!) 

thanks