SQL*Plus - Update field in a table using a variable

Hi,

Database Name: ess_test
Tablename: data_to_ess
Field that needs updated: status
Field that holds variable:  contract
Variable Name:  convar


Need a sample stored procedure or SQL*Plus command that would do the following:

Find all records where 'convar' equals 'C43' and 'status' equals 'Move_To_Planning'.  For all those records change the value in the 'status' field from 'Move_To_Planning' to 'Completed'.  Do nothing 'contract' field.  Can use sample login username and password for this example.

Is this enough information?

Sam