How do you resend a transaction without running a full node? The answers to the canonical "my transaction has been pending for too long" question require hundreds of GB of hard drive and bandwidth. I cannot meet such requirements to run a full node.

How can a 15 sat/B, 389 B Segwit unconfirmed transaction to Coinbase be replaced (transaction inputs double-spent?) with a higher transaction fee without downloading the entire blockchain on a full node?
You can use any software which is capable of broadcasting a transaction. This could be another wallet software such as Electrum or a command line tool like bitcoin-submittx.