To bulk delete transactions you can click on this link. Alternatively you can also follow the navigation as shown below.

Navigation: Bookkeeping > Client Specific > Sales / Purchases

                     Bookkeeping > Client Specific > Tasks > Journals

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Once you have followed the navigation, select the checkboxes for the transactions you would like to delete and a 'Delete' button will appear. 

Select 'Delete' and the following prompt will appear. 

Confirm you would like to delete the transactions by selecting 'Ok'. 

The same process is required to delete transactions from purchases and journals too.