5 Litres of emulsion paint usually comes in a plastic (polypropylene?) can nowadays. The can would be a nice little bucket without the rim for the lid...
You've got an old can - so just pull off the rim. This is easier if you saw through the rim from the inner edge - use any hacksaw, padsaw, pruning saw, or your best tenon saw
Finally, just crack out all the dried paint - what a cute little bucket, must be useful for something..?