hi, I've never done a pattern chain before but i just want to know if its possible to undo or erase it once its made?

Yes, it is. A Chain is literally, a play order list that instructs the sequencer in which order to assemble your various Pattern Sections into a linear structure. It is designed so that the composer can "try out" different arrangements of the musical sections. Patterns simply repeat forever... The Chain feature allows you to document exactly how many times each Section is to repeat, and where the transition to the next musical section occurs on a linear timeline.

Say you recorded four Sections A, B, C, D, where A is the main Verse, B is the Chorus, C is the Bridge, and D is the solo Section. In "trying out" arrangements you might want to see how AABACD works as a structure versus AABABCAD. Playback order can easily be reconfigured in the sequencer using the Chain feature.

You can create the Chain in realtime, while listening to the Pattern Sections play. When you record a CHAIN in realtime the Record Type is always "REPLACE" record. So to create (try out) a different arrangement of your Sections, simply record a new Chain.
You can create and/or edit the Chain while the sequencer is stopped, by using the Event List to manually insert commands.

Reconfiguring the playback order of Patter Sections is a definite concept of the CHAIN function, so, yes, you can freely and easily reconstruct your Chain.

The CHAIN can then be written out as a linear SONG using the Convert to Song feature. The Chain is the step between Pattern mode and Song mode.
Build your Pattern Sections. Chain the Sections... Then Convert the Chain to a linear Song.

After you create a Chain
Press [EDIT] to view the Event List
You will find options to edit and even convert to Song.
You can convert the Pattern Sections to as many Song locations as you desire, this means you can create several versions of the composition. Perhaps create and extended mix version, a shorter "radio" version, etc.

The SONG option allows you to target any measure in any Song location... This is how you can create extended mixes, medleys, etc., etc.

thanks for the detailed explanation.