It is not possible to convert a bank account to normal account (or vice-versa) per se.
However, we understand why you may want to do this - so below are some workarounds depending on your circumstances!
There are no entries or balances to date in the existing account
The simplest solution in this instance would be to delete the existing account and recreate it with the same details (Code/Account Name/Class) in the new format you require (i.e. add again as either a 'bank account' type or 'normal account' type.)
Do this via the Chart of Accounts:
There are entries or balances to date in the existing account
In this instance, it would not be possible to delete the existing account. Instead, we suggest you do the best alternative action as follows:
If you need to keep the same code for your new account, firstly change the code on the existing account to a new 'redundant' code (i.e. you won't want to use again)
Add the new account you want (either a 'bank account' type or 'normal account' type)
Reduce the old account balance to NIL by adding a single 'bank statement batch' entry line (either a payment or receipt amount) to the normal account to clear it
If there was a balance in the account to start, everything will then be switched and you will have the account in the format you require for future entries.
In the next period (after you close and roll forward this period, via 'Period end routine') you should be able to deactivate the old account.
It would not be possible to deactivate it in the current period if you performed the entry to remove any remaining balance. See below: