AccountsPortal automatically provides the necessary VAT/GST/Tax rates used in most countries. If the rates don't meet your requirements, you can edit the existing rates and/or create new rates. Typically you would add two entries for each percentage required. For example if you want to add in a 25% rate, you would normally create an entry for 25% with a Tax Type of 'Sales' (or 'Income') and and entry for 25% with a Tax Type of 'Purchases' (or 'Expenses').
Tax rates can be deleted at any time as long as if they have not been used in any transactions.
If the standard rate used in your country changes, then it is always recommended that you add a new rate rather than editing an existing rate. In some cases, the tax authority will expect you to apply the old rate for credit notes, refunds etc. Please read Manage a VAT/GST/Tax Rate Change for more information.