If your bet has been partially or is fully unmatched, you'll need to fix this bet. Fixing a partially matched bet is made easy using the Calculator with the Part lays toggle clicked to "Show".

If your bet is fully unmatched you can cancel the bet by clicking the X next to the bet, or hitting "Cancel unmatched above". You can then just update the lay odds in the calculator and relay the bet again.

If your bet is partially matched, this is when you can use the part lays option in the calculator. 

Your unmatched and partially matched bets will show differently depending on which exchange you're using. 

Smarkets: 
On the right-hand side, you'll see your activity and if the matched figure is in red, the bet has not been fully matched. 

Betfair Exchange: 
Again on the right-hand side with Betfair Exchange, you'll see your bets. If the bet hasn't matched, it will show as "Unmatched bet".

Matchbook: 
Also on the right-hand side, Matchbook will show the unmatched or partially matched bets under the unmatched tab like this.

This guide explains how to fix partially matched bets and how to avoid them.