Unfortunately, there is no process to directly unvoid or reverse a check in Keystone Client.
However, the check can be reissued using the Print Accounts Payable Checks (AP0007) process.
When the original check was voided, the associated invoice(s) were placed on hold. These invoices must be released for payment before the check can be reissued.
When reissuing the check, select the same check code and check date as the original check. For the check number, the system will default to the next available number from Check Code File Maintenance (LG0448). Override this value by entering the original check number with a leading zero.
Adding the leading zero allows the system to issue the check with a unique check number, while system reports will omit the leading zero, causing the check number to appear the same as the original.

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