How do I change the VAT rate on 1st April 2018?

If you have any outstanding batched transactions or invoices which still need to be processed for periods before the new rate is effective, then post these first.
Then, go to General|Tax Rates|List Tax Rates.
This will show you all your tax codes you currently have set up. You will need to find all the codes using the old rate and change them to the new rate where appropriate.
If you will be needing to back post at a later stage using the old rate, then you will need to create a new code called something like “Old VAT Rate @ 14%” to use for that purpose.