1. "Replace By Fee" or "RBF". If your wallet supports it, and you had it enabled, RBF allows you to effectively delete the transaction and send it again with a higher fee.
This only really works if the "change" from the transaction was sufficient to enable you to increase the miners fee, as you can't add new inputs, you can only use exactly the same inputs that were used in the first transaction. It is also only something that the "Sender" can do.
2. "Child Pays For Parent" or "CPFP". Basically, you use the "unspent output" or UTXO from the unconfirmed transaction, and use it in a new transaction with a MASSIVE fee that makes the average fee for BOTH transactions nice and big. Miner sees big fee, and confirms parent transaction so they can also confirm new transaction and claim fee.
This can work if you are the "Sender" if you control one of the addresses that an output got sent to (ie. a "change" address). This can also work if you are the "Receiver" and the sender messed up and used a low fee.