You can do so by opening the asset page you want to move and then tap on the 3 dots to the top right and select "Move to another portfolio".
Moving an asset moves all transactions as well as any linked transactions with it. Therefore if you have "deduct from..." or "add to..." options turned on, it will move those secondary transactions from other assets as well!
For example, if you move LTC and there is a buy transaction of LTC/ETH with "deduct from ETH holdings" enabled then, the sell transaction automatically created under Ethereum will also move with LTC.
Once you move a coin to another portfolio, the action is irreversible if the other portfolio already has some transactions of the same coin.
For example, if you move LTC from Portfolio A to B and Portfolio B already has some LTC transactions then transactions from Portfolio A will be merged into those of Portfolio B and there will be no way to separate these later.