Hello to everyone,
I have read some topics, but I still don't have very clear ideas. Yesterday I open a old electrum wallet (3.3.

to send a little bitcoin part to other wallet, but due to an error in entering fee the transaction is unconfirmed.
Which is the best solution to unlock it?
First, you should not use old Electrum version 3 and update to latest version 4.1.2 from official Electrum website.
Second, there is nothing to unlock here, but you can can read
THIS guide for canceling your transaction or changing your fee.
Always check Bitcoin mempool before sending transaction