Unstuck a unconfirmed transaction with Coinomi

It's generally not possible to cancel BTC transactions after they are broadcast to the network. If the transaction is valid, it can be held by miners until it's economically interesting for them to confirm the transaction (until there are no other transactions paying higher fees).

When you say "double spending", you are probably referring to a technique called "Child pays for parent" (CPFP). This involves making a new transaction spending the leftover change of the original transaction, but using a high fee. Miners can only confirm the child transaction after confirming the parent. By using high fees for the former, miners will be more incentivized into confirming the latter.

To do this in Coinomi, first check the original transaction on the block explorer. Unless you sent your full balance, you should see 2 destination addresses. One is the real destination, the other is your change. Take note of the change address (if there is no change, you can't do CPFP). Second, open your BTC wallet in Coinomi and at the top right corner of the screen select "... / Show unspent outputs". From the outputs list, select all addresses that are not your change address from the transaction and set them as "do not spend". Back to the BTC wallet in Coinomi, copy your own receive address, and make a transaction sending your full balance to it. Set "high" priority fees, or custom set a fee even higher than that if the transaction is urgent. After the new transaction confirms, reset your outputs back to spendable.

PS: Replace by fee (RBF) is a feature that allows increasing the fees of unconfirmed transactions without making a completely new transaction, and will be available in a future update.

PS2: you can contact support directly via support.coinomi.com