I did this last week for a 1600 (11.5.4 HF2) here is the link from F5 https://support.f5.com/kb/en-us/solutions/public/12000/800/sol12880.html
high level
- backup UCS with GUI
- backup the license key (at the tmsh prompt its mku -K)
- when you get the RMA you will likely need to install the right software (easies way to install the software is to get a trial license and activate the unit otherwise you have to do it all command line).
- once the software is the right version and HF, copy the UCS file (you must load the ucs file fromthe command prompt (tmsh load sys ucs [ucs file name] no-license , https://support.f5.com/kb/en-us/solutions/public/13000/100/sol13132.html)
- install the license
- install in the data center