david55 does has a point, if you have no experience with dial plan, you really should spend some time with the books.
If you are using a web base GUI and that is where the IVR is defined, it may be very easy to do with the GUI and very difficult to do with the config files. If you are not using a web GUI, perhaps these tips may help get you going.
In you zap or dahdi config files, look for a line that starts with ‘context =’. There may be several of them, you are interested in the one prior to a ‘channels =>’ line. That context defines where the inbound call starts processing in the extensions.conf file.
In extensions.conf, there may be lines that start with ‘#include’, this indicates another file that is read in as part of the extensions.conf, so there may be some searching that needs to be done.
Look for the context(s) you found above. they will the name you found enclosed in [ and ] (example: context=ivr — [ivr])
After that context label are extensions, starting with the text ‘exten =>’ , look for ‘exten => s,1,…’, that is where the inbound call will most likely start processing. From that point down, it is simply a script in a specific language. You are looking for any line that says something like… ‘wait(10)’.
Please understand, this is a really rough tutorial and making changes could be a dangerous thing of you do not know what you are doing. Make sure you make backups of any file you change.