You want to customise one of the layouts provided by JEvents but don't want these changes to be lost when you upgrade.  This is a very brief guide - if you want more general information about template overrides then I'd recommend you search for joomla template overrides in a search engine.

Basic Changes

Lets assume you want to customise the year list view of the Geraint layout.

1. Create a folder called html (if it doesn't already exist in your template folder) e.g. templates/ja_purity/html
2. Create a sub folder called com_jevents  e.g. templates/ja_purity/html/com_jevents
3. Create a sub folder called geraint e.g. templates/ja_purity/html/com_jevents/geraint
4. Create a sub folder called year e.g. templates/ja_purity/html/com_jevents/geraint/year
5. Copy the file listevents_body.php from the com_jevents layout folder to this new folder
6. Change this copy as you want (remember to clear your cache)

Joomla should use this copy in preference to the core JEvents component file from now on and you won't loose these changes if you upgrade JEvents

More Advanced Changes

You want to customise the stylesheets or images.

Follow setps 1-3 above and then

1. Create a folder called  assets/css or assets/images within this folder e.g. templates/ja_purity/html/com_jevents/geraint/assets/css
2. Put your modified stylesheet or image in this folder

Serious Changes

You realise you need to change or add a helper function (e.g. the iconic navigation bar)

Follow  steps 1-2 from "basic changes"then

1. create a sub folder called helpers
2. copy the helper file e.g. defaultviewnavtablebariconic.php to this folder
3. modify as required