Cause
By default, Oracle Forms uses memory mapped file i/o during runtime to read the compiled executables ( fmx and mmx files). This reduces the amount of memory required on a system when more than one user is running the same Form. The memory mapped file i/o mechanism, however, has the side effect of locking the forms in use preventing a developer from replacing them with a new version at Forms Runtime.
Solution
Memory mapped file i/o can be switched off by setting the FORMSxx_MMAP environment variable e.g

Forms 4.5.x : FORMS45_MMAP = 0
Forms 5.0.x : FORMS50_MMAP = 0
Forms 6.0.x : FORMS60_MMAP = 0
Forms 9.0.x : FORMS90_MMAP=FALSE
Forms 10.1.2 : FORMS_MMAP=FALSE
Forms 11.1.1 : FORMS_MMAP=FALSE

To switch memory mapped file i/o back on either unset FORMSxx_MMAP or explicitly set FORMSxx_MMAP to 1 (for Forms 4.5 - 6i) or TRUE (for Forms 9i / 10g / 11g)

When the FORMSxx_MMAP value to 0 or FALSE, Oracle Forms by-passes the default memory mapping and writes the file to memory for each time accessed. Thus, the executables are left closed on the file system, allowing for updates even during Forms Runtime. The disadvantage is that this may increase memory usage and reduce performance because the whole file structure is loaded into memory. For production systems, it is recommended that FORMSxx_MMAP should be unset or set to a value of 1 or TRUE

Important Note:
Setting FORMSxx_MMAP to 0 or FALSE only works for forms and menu executables i.e fmx and mmx files. It is not designed to work for libraries i.e pll or plx files, as mentioned in the Note:208566.1 Does FORMSxx_MMAP Variable Works For .plx Files ?
But there is a solution that works also for libraries, and quite convenient for production systems as FORMSxx_MMAP is not involved, it is based on using different directories in FORMS_PATH:
You will find it in the NOTE:286762.1 How to Replace Forms Runtime Files in Production Environments 'On The Fly'

Please note the following values may be used for FORMSxx_MMAP
(There are others but these are the most common)
For FALSE any of the following are expected work: 0 / FALSE / NO
For TRUE any of the following are expected to work: 1 / TRUE / YES

Note that FORMS(XX)_MMAP is set in the .env file which is normally the default.env.