The extend function extends the unlockTime(expiryTime) of the QBT lock for msg.sender. In order to call this function, the msg.sender should have locked some QBT into the locker.
QubitLocker
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functionextendLock(uint nextExpiry) external;
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nextExpiry : The next expiry time of the QBT lock for the msg.sender. This value should be larger than previous expiry time and smaller than maximum expiry time (i.e., current time + 2 years).