Merging is a Pro feature. Click Bulk action at the top of the Transactions page, tick between 2 and 50 transactions, and choose Merge. It isn't available when you use select all matching, because you need to tick the transactions yourself.
The transactions must share the same type, account (both accounts for a transfer), category and status; their dates can differ. Recurring transactions, debt or settlement payments, refunds and refunded transactions can't be merged. If the selection doesn't qualify, Merge is disabled and its tooltip tells you why. Transactions that already have a breakdown can be merged, and their items keep their names.
In the dialog, pick the transaction to keep — the earliest by default. Its note, tracking or cheque number and source carry over, and the date starts as its date but has its own field you can change. Each merged transaction becomes an item, named after its note or, when there isn't one, "Category · date"; you can rename them. The total, the combined commission and the tags are shown too: tags start as every tag from the selected transactions, and you can keep up to five. If other transactions have tracking or cheque numbers, a warning tells you they will be dropped.
Confirming merges everything at once or not at all, and your account balances stay exactly as they were. The other transactions are removed, and a merge can't be undone.