How to configurate a Text file

Hello, I'm a french engineer (sorry if my writing isn't clear enough) and I've a problem with Matlab (I precise I'm a beginner). In fact, I've to develop a script aimed at configurating a txt file.

-> First I've to find and save two values from a m.file (this step is okay !)

-> I've to set those values in a configuration file and I've problems at this step : How can replace a random value in a file. I've tested a few solutions but it doesn' word (such as strrep..) To be more clear I show you the file, the aim is at putting the saved value just after GISEMENT ='' Thank you!

      <?xml version="1.0" encoding="ISO-8859-1" standalone="yes"?><>
   <SESSION_MOCEM TYPE_ETUDE="0"/>
   <SCENARIO_MOCEM NOM_FICHIER_CIBLE="E:/Travail Alexandre SP2/Data_Pearl_Of_Seaway/Session_mocem/pearl_001/Pearl_of_Scandinavia29.3ds" NOM_FICHIER_VARIANTES="" NOM_FICHIER_TERRAIN="" NOM_VARIANTE="">
    <ZONE_VISEE X_POINT_VISE="3.103" GISEMENT="169" Z_POINT_VISE="31.426" DELTA_DISTANCE_1="90" MASQUAGE_HORS_ZONE="0" DELTA_AZIMUT_1="90" DELTA_DISTANCE_2="90" DELTA_AZIMUT_2="90" CLIPPING_FACE="0" Y_POINT_VISE="92.839"/>