Don't do that! Do not concatenate strings to build a SQL command. It leaves you wide open to accidental or deliberate SQL Injection attack which can destroy your entire database. Use Parametrized queries instead.
The chances are that your problem will disappear at the same time.
I'm not converting your actual query - that's your job - but it'll be along teh lines of:
using (SqlConnection con = new SqlConnection(strConnect))
using (SqlCommand com = new SqlCommand("UPDATE myTable SET myColumn1=@C1, myColumn2=@C2 WHERE Id=@ID", con))