Maybe
CurrentDb.Execute "UPDATE q01AssetTypes SET q01AssetTypes.PrefixA = '" & Forms!f01AssetsTypes.txtPrefix & "'"
Note that since there is no criteria there, all records will get that value, which may or may not be what you want:
display the code for every record.
It also assumes that the textbox value is a string and not a date. The form would have to be open and a value must be in the unbound control. I strongly suggest you do not run record modifying code that you are given on your real data - copy the table or db and play with the copy.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.