Formula to change part of a cell string ?

Dear folk

Dear folk

I am doing a labeling for an equipment if I want to change the first cell content for the below example to be say : Linksys.G001.F04.11,,,,,I need cell 5 to be G001.F04

And If I want to change content of first cell to be say : HP.G0AB.R01.18,,,,,,I need cell 5 to be G0AB.R01,,,,,,,etc

Linksys.R000.F03.10
EARTH CHASSIS
TO
RACK EARTH
R000.F03
PII234567

What formula do I need to do so ?

Thx