How can i convert varchar datatype to int type in sql

-- Table Creation
IF OBJECT_ID('TempDB..#RegDtls') IS NOT NULL DROP TABLE #RegDtls
CREATE TABLE #RegDtls(RegNo VARCHAR(50))
-- Sample Data
INSERT INTO #RegDtls(RegNo) VALUES('regi01001'),('regi01002')
-- Actual Data
SELECT RegNo FROM #RegDtls
-- Update Statement
UPDATE #RegDtls SET RegNo='regi'+RIGHT('00000'+CAST(SUBSTRING(RegNo,5,10)+1000 AS VARCHAR(10)),5)
-- Required Output
SELECT RegNo FROM #RegDtls