How to fill an existing column with random letters and numbers in SQL?

I'd like to populate an alraedy existing column with random values consisting of both letters and numbers. However, the distribution of letters and numbers should be the same for the whole column. Meaning it should look like that:

"VWE456GT7646" (e.g. 2 letters, 3 numbers, etc.) "FRS765TZ9854" "..."

I thought about an UPDATE statement, but don't know how it should exactly look like. Can somebody help me please?